JP4143179B2 - MRI equipment - Google Patents

Description

【００４６】
また、演算ユニット１０は、受信器８Ｒからシーケンサ５を介して送られてくるＭＲ信号のデジタルデータを入力してフーリエ空間（ｋ空間または周波数空間とも呼ばれる）への原データ（生データとも呼ばれる）の配置、および、原データを実空間画像に再構成するための２次元または３次元のフーリエ変換処理を行う一方で、画像データの合成処理を行うようになっている。なお、フーリエ変換処理はホスト計算機６に担当させてもよい。
【００４７】
この画像データの合成処理の好適な一例は、複数フレームの再構成画像データを対応画素毎に加算する処理、または、複数フレームの再構成画像データ間の対応するピクセル毎に最大値を選択する最大値投影（ＭＩＰ）処理である。なお、加算処理には、単純加算処理、加算平均処理、重み付け加算処理などが含まれる。また、この合成処理の別の例として、フーリエ空間上で複数フレームの軸の整合をとって原データのまま１フレームの原データに合成するようにしてもよい。
【００４８】
記憶ユニット１１は、原データおよび再構成画像データのみならず、上述の合成処理が施された画像データを保管することができる。表示器１２は画像を表示する。また入力器１３を介して、術者が希望する同期タイミング選択用のパラメータ情報、スキャン条件、パルスシーケンス、画像合成法などの情報をホスト計算機６に入力できるようになっている。
【００４９】
また、息止め指令部として音声発生器１９を備えている。この音声発生器１９は、ホスト計算機６から指令があったときに、息止め開始および息止め終了の例えばメッセージを音声として発することができる。
【００５０】
さらに、心電計測部は、患者Ｐの体表に付着させてＥＣＧ信号を電気信号として検出するＥＣＧセンサ１７と、このセンサ信号にデジタル化処理を含む各種の処理を施してホスト計算機６およびシーケンサ５に出力するＥＣＧユニット１８とを備える。この心電計測部による計測信号は、ＥＣＧ−ｐｒｅｐスキャンと心電同期イメージングスキャンを実行するときにホスト計算機６およびシーケンサ５により用いられる。これにより、心電同期法の同期タイミングを適切に設定でき、この設定した同期タイミングに拠る心電同期イメージングスキャンを行ってＭＲ原（生）データを収集できるようになっている。
【００５１】
次に、同期タイミングを事前設定するための処理を、図３〜図６を参照して説明する。
【００５２】
ホスト計算機６は、図示しない所定のメインプログラムを実行している中で、入力器１３からの指令に応答して、図３に示すＥＣＧ−ｐｒｅｐスキャンの実行ルーチンを開始する。
【００５３】
最初に、ホスト計算機６は、ＥＣＧ−ｐｒｅｐスキャンを実行するスキャン条件およびパラメータ情報を入力器１３から読み込む（同図ステップＳ１）。スキャン条件には、スキャンの種類、パルスシーケンスの種類、位相エンコード方向などが含まれる。パラメータ情報には、心電同期タイミングを決めるための初期時間Ｔｏ（ここでは、ＥＣＧ信号中のＲ波のピーク値からの遅延時間）、時間増分の刻み幅Δｔ、回数カウンタＣＮＴの上限値などが含まれ、これらのパラメータは操作者により任意に設定できる。
【００５４】
次いで、ホスト計算機６は、シーケンスの実行回数をカウントする回数カウンタＣＮＴ、同期タイミングを決めるための時間の増分パラメータＴｉｎｃ、および遅延時間ＴDLをクリヤする（ＣＮＴ＝０，Ｔｉｎｃ＝０，ＴDL＝０：ステップＳ２）。この後、ホスト計算機６は音声発生器１９にメッセージデータを送出して、例えば「息を止めて下さい」といった息止め指令を患者Ｐに対して行わせる（ステップＳ３）。この息止めは、ＥＣＧ−ｐｒｅｐスキャン実行中の患者の体動を抑制する上で実施する方が好ましいが、場合によっては、息止めを実施しない状態でＥＣＧ−ｐｒｅｐスキャンを実行するようにしてもよい。
【００５５】
このように準備が整うと、ホスト計算機６はステップＳ４以降の処理を順次実行する。これにより、心電同期タイミングを変更しながらのスキャン実行に移行する。
【００５６】
具体的には、Ｒ波のピーク到達時間からの遅延時間ＴDLが、ＴDL＝Ｔｏ＋Ｔｉｎｃにより演算される（ステップＳ４）。次いで、ＥＣＧユニット１８で信号処理されたＥＣＧ信号が読み込まれ、その信号中のＲ波のピーク値が出現したか否かが判断される（ステップＳ５）。この判断処理はＲ波出現まで繰り返される。Ｒ波が出現すると（ステップＳ５，ＹＥＳ）、ステップＳ４で演算したその時点の遅延時間ＴDLがＲ波ピーク時間から経過したかどうかが続いて判断される（ステップＳ６）。この判断処理も遅延時間ＴDLが経過するまで続けられる。
【００５７】
Ｒ波のピーク時刻から遅延時間ＴDLが経過すると（ステップＳ６，ＹＥＳ）、各回のスキャンのパルスシーケンスの開始をシーケンサ５に指令する（ステップＳ７：図４参照）。このパルスシーケンスは、その種類としては、後述するイメージング用のパルスシーケンスと同一に設定するのが望ましい。ただし、このＥＣＧ−ｐｒｅｐスキャンの目的は、撮像対象の信号強度が最高になる心電同期用の遅延時間を設定することであるから、心電同期イメージングスキャンを３次元で行う場合であっても、このＥＣＧ−ｐｒｅｐスキャンは、撮像対象を含んでさえすれば２次元で行えばよい。この２次元スキャンにより全体の撮像時間を最小限に止めることができる。心電同期イメージングスキャンが２次元の場合は、ＥＣＧ−ｐｒｅｐスキャンは２次元スキャンでよく、またイメージングスキャンよりも多少、空間分解能（マトリクスサイズなど）を下げてもよい。
【００５８】
このＥＣＧ−ｐｒｅｐスキャンとして、高速ＳＥ法とハーフフーリエ法とを組み合わせた２次元ＦＡＳＥ（２Ｄ−ＦＡＳＥ）法が使用される。勿論、このスキャン法の代わりに、通常のＳＥ法、高速ＳＥ法、ＥＰＩ法、ＦＥ法、セグメンテッドＦＦＥ法など、各種の２次元スキャン法を採用することもできる。
【００５９】
このシーケンス開始指令に応答し、シーケンサ５は操作者から指令された種類のパルスシーケンスの実行を開始するので、患者Ｐの所望部位の断面が２次元スキャンされる。
【００６０】
上記シーケンス実行開始の指令後、回数カウンタＣＮＴ＝ＣＮＴ＋１の演算が行われ（ステップＳ８）、さらに、時間の増分パラメータＴｉｎｃ＝ΔＴ・ＣＮＴの演算が行われる（ステップＳ９）。これにより、パルスシーケンスの実行を指令した各回毎に回数カウンタＣＮＴのカウント値が１ずつ増加し、また同期タイミングを調整する増分パラメータＴｉｎｃがそのカウント値に比例して増加する。
【００６１】
次いで、各回のパルスシーケンスの実行に必要な予め定めた所定期間（例えば後述する撮像条件では７００ｍｓｅｃ程度）が経過するまで、そのまま待機する（ステップＳ１０）。さらに回数カウンタＣＮＴ＝予め定めた上限値になったか否かを判断する（ステップＳ１１）。同期タイミングを最適化させるために、遅延時間ＴDLを各種の時間値に変更しながら、例えば５枚の２次元像を撮影する場合、回数カウンタＣＮＴ＝５に設定される。回数カウンタＣＮＴ＝上限値に到達していない場合（ステップＳ１１，ＮＯ）、ステップＳ４の処理に戻って上述した処理が繰り返される。反対に、回数カウンタＣＮＴ＝上限値に到達した場合（ステップＳ１１，ＹＥＳ）、息止め解除の指令が音声発生器１９に出され（ステップＳ１２）、その後の処理はメインプログラムに戻される。息止め解除の音声メッセージは例えば「息をして結構です」である。
【００６２】
上述の処理を順次実行すると、一例として、図４に示すタイミングで準備用のパルスシーケンスが各回毎に実行される。例えば、初期時間Ｔｏ＝３００ｍｓｅｃ，時間刻みΔＴ＝１００ｍｓｅｃを指令していたとすると、第１回目のシーケンスに対する遅延時間ＴDL＝３００ｍｓｅｃ、第２回目のそれに対する遅延時間ＴDL＝４００ｍｓｅｃ、第３回目のそれに対する遅延時間ＴDL＝５００ｍｓｅｃ、…といった具合に同期タイミングを決する遅延時間ＴDLが調整される。このため、息止め指令後の最初のＲ波がピーク値に達すると、その到達時刻から遅延時間ＴDL（＝Ｔｏ）後に、例えば２Ｄ−ＦＡＳＥ法に基づく第１回目のスキャンＥＣＧｐｒｅｐ１が所定時間（約７００ｍｓｅｃ）継続し、エコー信号が収集される。このシーケンス継続中に次のＲ波が出現した場合でも、図３のステップＳ１０の待機処理があるので、このＲ波出現には何等関与されずに、シーケンスは続けられる。つまり、ある心拍に同期して開始されたシーケンスの実行処理は次の心拍にまたがって続けられ、エコー信号が収集される。
【００６３】
そして、回数カウンタＣＮＴが所定値に到達していない場合、図３のステップＳ５〜ステップＳ１１の処理が再び実行される。このため、図４の例では、３番目のＲ波が出現してピーク値に達すると、この到達時点から遅延時間ＴDL＝Ｔｏ＋Ｔｉｎｃ＝４００ｍｓｅｃが経過した時点で、第２回目のスキャンＥＣＧｐｒｅｐ２が所定時間継続し、同様にエコー信号が収集される。このスキャンが終わって次のＲ波が出現すると、遅延時間ＴDL＝Ｔｏ＋２・Ｔｉｎｃ＝５００ｍｓｅｃが経過すると、第３回目のスキャンＥＣＧｐｒｅｐ３が所定時間継続し、同様にエコー信号が収集される。さらに、このスキャンが終わって次のＲ波が出現すると、遅延時間ＴDL＝Ｔｏ＋３・Ｔｉｎｃ＝６００ｍｓｅｃが経過すると、第４回目のスキャンＥＣＦｐｒｅｐ３が所定時間継続し、同様にエコー信号が収集される。このスキャンが所望回数、例えば５回続き、合計５フレーム（枚）の同一断面のエコーデータが収集される。
【００６４】
各フレームのエコーデータは順次、シーケンサ５を経由して演算ユニット１０に送られる。演算ユニット１０は周波数空間のエコーデータを２次元フーリエ変換法により実空間の画像に再構成する。この再構成画像はＭＲＡ像データとして記憶ユニット１１に記憶される。ホスト計算機６は、例えば入力器１３からの操作信号に応答して、このＭＲＡ像を順次、シネ（ＣＩＮＥ）表示する。
【００６５】
このように、ダイナミックに心電同期の遅延時間（同期タイミング）が変更された複数枚のＭＲＡ像の表示例を図５（ａ）〜（ｅ）に示す。これらの図は実際の画像写真を模写したもので、２Ｄ−ＦＡＳＥ法（実効ＴＥ（ＴＥeff ）＝４０ｍｓ，エコー間隔（ＥＴＳ）＝５ｍｓ，ショット数＝１，スライス厚（ＳＴ）＝４０ｍｍ，スライス枚数（ＮＳ）＝１，加算枚数（ＮＡＱ）＝１，マトリクスサイズ＝２５６×２５６，ＦＯＶ＝４０×４０ｃｍ，実際のスキャン時間＝７００ｍｓ程度）、かつ、位相エンコード方向＝図の上下方向（体軸方向）に設定して実験した肺野の画像写真を模式的に表している。この画像で目的としているエンティティとしての血流は下行大動脈である。同図において遅延時間ＴDLはそれぞれ、（ａ）でＴDL＝３００ｍｓｅｃ，（ｂ）でＴDL＝４００ｍｓｅｃ，（ｃ）でＴDL＝５００ｍｓｅｃ，（ｄ）でＴDL＝６００ｍｓｅｃ，（ｅ）でＴDL＝７００ｍｓｅｃ、となっている。
【００６６】
これらのシネ表示像を目視観察すれば、大動脈流からのエコー信号が最も強く表れ、かつ大動脈全体が明瞭なのは、同図（ｅ）のＭＲＡ像である。ほかの（ａ）〜（ｄ）のＭＲＡ像の場合、（ｅ）に比べて、大動脈流の写っている範囲が極く一部または短い範囲であって、拍動に伴う血流の速度が速いなどの要因から、エコー信号の強度が相対的に低く、フローボイド現象に近い状態になっている。つまり、肺野において大動脈流のＭＲＡ像を得る場合、この実験の場合には、同図（ｅ）の状態、すなわち遅延時間ＴDL＝７００ｍｓｅｃが最適となる。これにより、心電同期の同期タイミングは、Ｒ波のピーク到達時刻から遅延時間ＴDL＝７００ｍｓｅｃ後の時刻ということが判明する。
【００６７】
したがって、操作者は、このように遅延時間ＴDLをダイナミックに変えて撮像した複数枚のＭＲＡ像から最適な画像、すなわち最適な遅延時間ＴDLを目視判定で決し、この遅延時間のパラメータを引き続き行うイメージングスキャンに手動で反映させる処理を行う。なお、ここでの「最適な」の用語は、同期タイミングの与えられた設定法の条件下で「最も適切と思われる」を意味する。
【００６８】
さらに、上述したＥＣＧ−ｐｒｅｐスキャンにおいて、位相エンコード方向を大動脈流の走行方向に沿った方向（体軸方向）に意図的に設定している。これにより、位相エンコード方向をそれ以外の方向に設定した場合に比べて、大動脈流の走行方向情報（方向性）を欠落または落とさずに、より明瞭に撮像することができ、その描出能に優れている。この理由を以下に述べる。
【００６９】
一般に、肺血管や肝臓の血管（門脈）に代表される血流はＴ₂ 時間が若干短い（Ｔ₂ ＝１００〜２００ｍｓ）ことが知られている。このＴ₂ 時間の短めの血流は、Ｔ₂ 時間が長いＣＳＦや関節液（Ｔ₂ ＞２０００ｍｓ）に比べて、信号の半値幅が広がることが分かっている。このことは、例えば、文献「R. Todd Cons- table and John C. Gore, "The loss of small objects in Variable TE ima- ging: Implications for FSE, RARE, and EPI", Magnetic Resonance in Medi- cine 28, 9-24, 1992 」に示されている。同文献によると、Ｔ₂ 時間の異なる物質に対する信号値の広がりは、図６に示すように、“point spread function ”によって表される。同図のグラフは、静磁場＝１．５Ｔ、ＴＥeff ＝２４０ｍｓ、エコー間隔（ＥＴＳ）＝１２ｍｓのときのもので、横軸が位相エンコード方向の画像上の画素数を表し、縦軸が任意単位の信号強度である。これによると、Ｔ₂ ＝２０００ｍｓのＣＳＦや関節液に比べて、Ｔ₂ ＝２００ｍｓの血液（動脈）はその半値幅が広がっている。これは、Ｔ₂ ＝２００ｍｓの血液（動脈）はＣＳＦや関節液よりも、見掛け上、位相エンコード方向の幅が伸びているのと等価であると言える。したがって、Ｔ₂ ＝２００ｍｓの血液（動脈）は、ＣＳＦや関節液に比べて、画像全体が位相エンコード方向に余計にぼけることを示している。
【００７０】
そこで、位相エンコード方向を血流方向に設定することで、Ｔ₂ 時間が短い血液の位相エンコード方向の信号値のピクセル上の広がり（ぼけ）の度合いが、Ｔ₂ 時間が長いものよりも大きいことを積極的に利用でき、血流方向が強調されるのである。したがって、上述したように、心電同期のための最適な（適切な）ＭＲＡ像（すなわち最適な（適切な）遅延時間）を選択するときに、その選択がより容易化される。
【００７１】
次に、この実施形態の心電同期イメージングスキャンの動作を図７〜図１０を参照して説明する。
【００７２】
ホスト計算機６は、入力器１３からの操作情報に応答して図７に示す処理を実行する。
【００７３】
これを詳述すると、ホスト計算機６は、前述したＥＣＧ−ｐｒｅｐスキャンを通して操作者が決めた最適な（適切な）遅延時間ＴDLを入力器１３を介して入力する（ステップＳ２０）。次いで、ホスト計算機６は操作者が入力器１３から指定したスキャン条件（画像サイズ、スキャン回数、スキャン間の待機時間、スキャン部位に応じたパルスシーケンスなど）および画像合成処理法の情報（再構成画像での合成か周波数空間上での合成か、加算処理か最大値投影（ＭＩＰ）処理かなど。加算処理の場合には、単純加算、加算平均処理、重み付け加算処理のいずれかなど）を入力し、それらの情報を制御情報に処理し、その制御情報をシーケンサ５および演算ユニット１０に出力する（ステップＳ２１）。
【００７４】
なお、ホスト計算機６は、このステップＳ２１の処理において、画像合成を達成するためのスキャン回数（すなわち同一撮像部位に何枚の画像を撮像するか）に応じて、自動的に位相エンコード方向の変更角度を演算し、スキャン毎の位相エンコード方向の角度変更情報をパルスシーケンスに組み込んでシーケンサ５に送るようになっている。この角度変更情報は例えば、画像合成を行う画像枚数が２枚の場合、１回目のスキャンが終わって２回目のスキャンを実行するときに、位相エンコード方向を１回目のそれから９０°変える、というものである。
【００７５】
次いで、スキャン前の準備完了の指示があったと判断できると（ステップＳ２２）、ステップＳ２３で息止め開始の指令を音声発生器１９に出力する（ステップＳ２３）。これにより、音声発生器１９は、ＥＣＧ−ｐｒｅｐスキャン時と同様に「息を止めて下さい」といった内容の音声メッセージを発するから、これを聞いた患者は息を止めることになる（図９参照）。
【００７６】
この息止め開始を指令した後、ホスト計算機６は所定の調整時間Ｔsp（例えば１秒）の間そのまま待機し、患者が完全に息止め状態に移行したタイミングを見計らう（ステップＳ２４）。
【００７７】
この調整時間の待機が完了すると、ホスト計算機６はＥＣＧ信号に関する処理を順次実行する（ステップＳ２５〜Ｓ２７）。まず、ＥＣＧ信号を入力し、その信号にＲ波のピーク値が出現するまでＥＣＧ信号を監視しながら待機する。Ｒ波が出現し、そのピークに達すると、そのピーク到達時刻からステップＳ２０で読み込んだ遅延時間ＴDLだけ待機する処理を行う（ステップＳ２７）。この遅延時間ＴDLの値は、前述したようにＥＣＧ−ｐｒｅｐの事前スキャンにより対象とする血流や組織を撮像する上で最もエコー信号の強度が高くなり、撮像対象の描出能に優れる値に最適化されている。
【００７８】
なお、息止め指令から最初のＲ波出現までの調整時間Ｔsp′は、上述した値Ｔspに、この時間Ｔsp経過後からＲ波出現までの任意時間βを加えた値になる。
【００７９】
この最適な遅延時間ＴDLが経過した時点が心電同期タイミングであるとして、ホスト計算機６はシーケンサ５にスキャンの開始を指令する（ステップＳ２８）。この指令を受けたシーケンサ５は、既に送られて記憶していたパルスシーケンス情報に応じて送信器８Ｔおよび傾斜磁場電源４を駆動し、イメージングスキャンを実行する。このスキャン処理の一例を図８に、そのタイミングを図９にそれぞれ示す。
【００８０】
図８に示す処理例はスキャンの回数が２回であって、後述する画像合成処理は２枚の再構成画像を相互に加算処理するものである。このスキャン制御例を説明する。
【００８１】
シーケンサ５は通常、ホスト計算機６からイメージングスキャンの開始指令が送られてきたか否かを判断しながら待機している（ステップＳ２８−１）。スキャンが指令されると、シーケンサ５は、指令されている位相エンコード方向に基づく１回目のスキャンを実行する（ステップＳ２８−２）。この１回目のスキャンの場合、例えば２Ｄ−ＦＡＳＥ法が選択され、また位相エンコード方向がＺ軸方向に、読出し方向（周波数エンコード方向）がＸ軸方向に各々設定されている（図９参照）。この結果、例えば、肺野のスキャンに伴う１フレーム分のＭＲ原データ（生データ）が収集される。
【００８２】
このとき２Ｄ−ＦＡＳＥ法によって患者Ｐから発生したエコー信号は、ＲＦコイル７で受信され、受信器８Ｒに送られる。受信器８Ｒではエコー信号に各種の前処理が施され、デジタル量に変換される。このデジタル量のエコーデータは演算ユニット１０に送られ、内蔵メモリに拠る例えば２次元ｋ空間に配置される。このｋ空間上のエコーデータの組は適宜なタイミングで例えば２次元フーリエ変換して実空間断層像に変換される。この再構成画像データは記憶ユニット１１に一時的に格納されて２回目のスキャンを待つ。
【００８３】
シーケンサ５は１回目のスキャン指令後、そのスキャンが完了したか否かを判断しながら待機している（ステップＳ２８−３）。
【００８４】
この後、シーケンサ５は２回目のスキャンまで所定時間Ｔｗの間待機する（ステップＳ２８−４）。この待機時間Ｔｗは、１回目のスキャンに拠る原子核スピンの挙動が励起パルス印加前の定常状態まで戻るまで待つことを意図したものである。これにより、２回目のスキャン時の原子核スピンの挙動が１回目のそれに殆ど影響されないから、Ｔ１緩和時間の多少の影響が少なくなり、２回のスキャンに伴うエコーデータのばらつきが少なくなる。この待機時間Ｔｗとしては、例えば６秒程度のオーダである。なお、術者が入力器１３を介して待機時間Ｔｗの長短を調節するようにすることも、望ましい態様の１つである。
【００８５】
この待機時間Ｔｗが経過すると、シーケンサ５はＥＣＧ信号を入力し、その中のＲ波ピークの出現を待つ（ステップＳ２８−５，６）。このため、実際の待機時間Ｔｗ′は指定待機時間Ｔｗに、この時間Ｔｗ経過からＲ波出現までの任意時間βを加えた値になる。Ｒ波が出現すると、再び、最適化された遅延時間ＴDLの間は待機状態となる（ステップＳ２８−７）。
【００８６】
この遅延時間ＴDL経過後に、シーケンサ５は２回目のスキャンを１回目と同じスライスについて同様に実行する（ステップＳ２８−８）。ただし、このときの位相エンコード方向は、予め設定されている角度だけ変更されてスキャンが実施される。例えば、１回目の位相エンコード方向から９０°ずれた方向に、２回目のスキャン時の位相エンコード方向が設定されている。一例として、位相エンコード方向がＸ軸方向に、読出し方向（周波数エンコード方向）がＺ軸方向に各々変更される。このエンコード状態で２回目のスキャンが実施され（図９参照）、収集されたエコー信号の処理は１回目のときと同じである。
【００８７】
そして、シーケンサ５は２回目のスキャン完了が判断できると、スキャン完了の通知をホスト計算機６に対して行う（ステップＳ２８−９，１０）。
【００８８】
図７のステップＳ２９において待機していたホスト計算機６は、シーケンサ５からのスキャン完了通知を受ける。これにより、ホスト計算機６は息止め解除の指令を音声発生器１９に出力する（ステップＳ３０）。このため、音声発生器１９は、例えば「息をして結構です」といった音声メッセージを患者に向けて発し、息止め期間が終わる（図９参照）。
【００８９】
この一連のデータ収集処理が終わると、ホスト計算機６は、演算ユニット１０に対して記憶ユニット１１に一次格納されている２回のスキャンに拠る再構成画像Ａ，Ｂの合成処理および表示を指令する（ステップＳ３１）。この合成処理の方法は前記ステップＳ２１の入力処理で認識できているから、その方法で画像合成を行って１枚の合成画像Ｃを生成する。合成処理法としては、いまの場合、２枚の画像Ａ，Ｂを画素値毎に加算する加算処理や、２枚の画像Ａ，Ｂの最大値投影処理が使用できる。加算処理の場合、単純加算、加算平均、重み付け加算のいずれかの方法が指令されているので、その方法に沿って行う。この結果、図９に模式的に示す如く、２枚の再構成されたＭＲＡ画像Ａ，Ｂから合成画像Ｃが得られる。
【００９０】
このように本実施形態によれば、心電同期イメージングスキャンを行うときの同期タイミング（上述した遅延時間ＴDL）が複数の心拍を利用して適切に設定されるので、血流などの撮像対象が発生するエコー信号が最も高くなる状態でイメージングスキャンが実施される。血流速度が相対的に遅かったり、フローボイド現象に因ってエコー信号の強度が相対的に低下または殆ど零となる状態を確実に回避できる。最適化された同期タイミングで安定した、高描出能のＭＲＡ像を提供することができる。また高速ＳＥ系のパルスシーケンスを使用すると、サスセプタビリティや形態の歪みの点での優位性も当然に享受することができる。
【００９１】
これを従来の心電同期法と比較する。従来のように、心電図波形の例えばＲ波のピーク時間から常に一定の遅延時間経過後に、イメージングシーケンスを実行する場合、Ｒ波出現の直後に発生する乱流的な血流時間帯を回避し、血流状態が比較的安定している時間帯を選択してスキャンできる。これにより、乱流的な血流の影響を排除でき、安定した血流状態でのエコー信号を周波数空間の位相エンコード方向の中心域に配置して、再構成した画像のコントラストを高めることはできる。
【００９２】
しかし、一定の遅延時間がいつも妥当とは限らず、例えば、心電同期法を使って横緩和時間が短めの組織や血流などを撮像する場合でも、被検体の固体差や撮像部位の違い、さらにはパルスシーケンスの種類に応じて適正な同期タイミングが存在する筈である。この同期タイミングに過不足があると、心筋から駆出された血液の流れが未だ撮像部位まで十分には到達していなかったり、その反対に駆出血液の流れが既に撮像部位を抜けてしまい、エコー信号が生じないフローボイド（flow void ）現象が発生してしまう。
【００９３】
このような撮像時間の比較的長いＳＥ系のシーケンスと心電同期法を併用して横緩和時間が短めの組織や血流などを撮像する場合、エコー信号値が最強になる同期タイミングの最適化を考慮した手法が従来では知られていなかったが、本発明により、そのような事態を解消できる。１回の励起に伴う撮像時間が長いため、１心拍内で同期タイミング（時相）変えて複数回スキャンすることは実際上困難であるが、本発明のように複数心拍にわたってＥＣＧ−ｐｒｅｐスキャンを考え、その内の複数のＲ波に別々の同期タイミングを割り当てることで、ダイナミックに同期タイミングを変えてスキャンできる。この結果、同期タイミングの相違を反映し、比較的コントラストの良い、しかも、Ｒ波出現直後の乱流発生の影響を回避した準備用ＭＲＡ像が複数枚得られる。この画像を用いて同期タイミングを事前に最適に（適切に）設定でき、したがって上述した各種の効果を享受することができる。
【００９４】
また、このように同期タイミングが予め最適化されるから、撮像のやり直しを行う必要も殆ど無くなり、操作者の操作上の負担も軽減するとともに、患者スループットの向上も可能になり、さらに患者の負担も軽減または抑制される。
【００９５】
ところで、本実施形態によれば、位相エンコード方向を変えて収集したエコーデータの複数枚の画像から新規な合成画像を得ることができる。この合成画像は位相エンコード方向の変更制御に拠って、とくに、Ｔ₂ 緩和時間の短めな血流の描出能に優れている。
【００９６】
この理由は、前記図６で説明した位相エンコード方向の画素値の強調（ぼけ）の効果を積極的に利用し、この強調効果を得た画像を複数枚合成することにある。これを図１０で模式的に説明する。同図に示すように、血管Ｂ１からその直交方向に枝分かれした血管Ｂ１１があって、例えば１回目のスキャン時の位相エンコード方向が血管Ｂ１の走行方向に略一致し、２回目のスキャン時の位相エンコード方向が枝分かれした血管Ｂ１１に略一致しているとする。同図（ａ）に示すように、１回目のスキャンに拠る位相エンコード方向の信号値の広がりに拠って各画素が疑似的に伸びたものと等価になり、その位相エンコード方向と略一致している血管Ｂ１はぼけに因って強調され、反対に、これに直交する方向の血管Ｂ１１はぼけてしまう。しかし、２回目のスキャンでは位相エンコード方向が９０°変更されるので、今度は反対に同図（ｂ）に示すように、一方の血管Ｂ１はぼけるが、もう一方の血管Ｂ１１はぼけに因って強調される。
【００９７】
上述した実施形態では、同図（ａ）および（ｂ）の再構成画像が画素毎に加算（合成）されるので、同図（ｃ）に示す如く、両方の位相エンコード方向の血流Ｂ１，Ｂ１１の画像が共に消失されずに残る。しかも、位相エンコード方向にぼけるとはいえ、加算処理の場合には、２枚の画像を画素毎に加算しているからアベレージング法の利点も享受でき、併せて血流の信号値を上げ、Ｓ／Ｎを向上させる。図１０では直交する２方向についてのみ説明したが、血流Ｂ１が１回目の位相エンコード方向から多少ずれていても、また血流Ｂ１１が２回目のそれから多少ずれていても、かかる利点を多少とも享受できる。したがって、縦横無尽に走行している肺血管などの血管に対し、その走行方向情報を殆ど欠落させることなく、高いＳ／Ｎおよび実質部の高いコントラストで描出することができ、診断能の向上に寄与可能になる。
【００９８】
従来の位相エンコード方向が固定のアベレージング法の場合には、Ｓ／Ｎ比向上は見込めるものの、例えば図１０（ａ）に示す方向に位相エンコード方向を設定したときには、血流Ｂ１１が位相エンコード方向のぼけに因って目視で識別困難になるか、または、消失してしまうことがあった。また同図（ｂ）に示す方向に位相エンコード方向を設定したときには、血流Ｂ１が同様の問題に直面していた。しかしながら、本実施形態によって、そのような状態を回避し、とくに、肺野や肝臓の血管などＴ₂ 時間が短めの血管についてその走行方向の情報量を低下させることなく描出することができるようになった。
【００９９】
さらに、上述した実施形態の場合、１回の息止め期間に２回全部のイメージングスキャンを終えるようにしている。このため、肺などの周期的運動による体動アーチファクトの発生を抑制できるとともに、複数回にわたって息止め撮像をするときの患者の体自体の位置ずれに因る体動アーチファクトの発生も合わせて低減できる。これにより、アーチファクトのより少ない高品質のＭＲ像を提供できる。
【０１００】
また、２回のスキャンの間にスピンの回復を待つ待機時間を設定しているから、２回目のスキャンもより的確に実行でき、高品質の画像を提供できる。
【０１０１】
さらに、そのような待機時間を設定したとしても、多くの場合、１回目、２回目のスキャンは１．５秒程度、待機時間は４秒程度で済むので、息止めの期間は６秒ちょっとで済む程度である。したがって、患者の１回の息止めの継続時間は短くて済み、子供や高齢者にとっても、息止めに関する精神的、体力的負担が軽いという利点もある。
【０１０２】
第２の実施形態
本発明の第２の実施形態を説明する。なお、この第２〜第５の実施形態は、上述のＥＣＧ−ｐｒｅｐスキャンを通して最適化された遅延時間ＴDLを用いる心電同期イメージングスキャンのほかの例に関する。
【０１０３】
前述した第１の実施形態のイメージングスキャンは、位相エンコード方向を変えて合計２回のスキャンを行うものであったが、本発明はこれに限定されるものではない。そこで、この第２の実施形態のＭＲＩ装置では、図１１に示す如く、位相エンコード方向を変えて合計４回のイメージングスキャンを順次、所定待機時間毎に実施し、これにより４５°ずつ位相エンコード方向が異なった４フレーム分のＭＲ原データを得る。各回のイメージングスキャンは心電同期法としてのＥＣＧゲート法を採用し、その同期タイミングは、事前に実施されるＥＣＧ−ｐｒｅｐスキャンを通して最適化された遅延時間ＴDLで決められる。画像処理の一例としては、原データをそれぞれのフレームで画像再構成し、４枚の再構成画像を合成処理（加算処理または最大値投影処理）を行う。これによっても、上述した実施形態のものと同等またはそれ以上に、位相エンコード方向のより細かい角度制御に拠って、血管の走行情報が豊富なＭＲ画像を得ることができる。
【０１０４】
すなわち、加算（合成）する画像枚数ｎ（すなわち位相エンコード方向の変更回数）はｎ≧２であればよい。
【０１０５】
第３の実施形態
第３の実施形態を図１２〜１３を参照して説明する。この実施形態は３次元の心電同期イメージングスキャンに関する。
【０１０６】
この３次元の心電同期イメージングスキャンの場合、スライス方向を不変とした状態で、位相エンコード方向と読出し方向をスキャン毎に交換しながら（インターリーブさせながら）、複数個のスライスエンコード量に応じた複数回のスキャンが実行される。具体的な一例として、図１２に、ホスト計算機６およびシーケンサ５によって指令される心電同期イメージングスキャンのシーケンス（前述したＥＣＧ−ｐｒｅｐスキャンの後で実施される）の一例を示す。各スライスエンコード量に応じたスキャンでは、位相エンコード方向を変える手法のほか、ＥＣＧゲート法および息止め法が採用されている。心電同期タイミングは、事前に実施されるＥＣＧ−ｐｒｅｐスキャンを通して最適化された遅延時間ＴDLで決められる。例えば、図１３（ａ），（ｂ）に示す如く、腹部を３次元撮像する場合のボリューム領域のデータ収集は、ＲＬse1 ，ＨＦse1 ，ＲＬse2 ，ＨＦse2 ，…，ＲＬsen ，ＨＦsen の順序で各スライスエンコード量に応じたスキャンが２ｎ回（ｎは２以上の整数）、例えば３Ｄ−ＦＡＳＥ法で実施される。
【０１０７】
スキャンＲＬseまたはＨＦseは、ボリューム領域の３次元原データを提供するスライスエンコード傾斜磁場による各スライスエンコード量に対する心電同期のシングルスキャンを表す。しかし、スキャンＲＬseとスキャンＨＦseでは位相エンコード方向が異なる。スキャンＲＬseの場合、図１３（ｂ）の実線矢印Ｘ１で示すように、位相エンコード方向が患者の体の左右ＲＬ方向に設定される。これに対し、スキャンＨＦseの場合、同図の点線矢印Ｘ２で示すように、位相エンコード方向は患者の上下（頭部／脚部）ＨＦ方向に設定され、左右方向とは９０度異なる。添字ｓｅ１…ｓｅｎは、各スキャンに対するスライスエンコードの傾斜磁場量を表す。この例示シーケンスでは、同一のスライスエンコード量ｓｅ１（…ｓｅｎ）について第１、第２の２回の心電同期スキャンが実施され、この１組のスキャンがスライスエンコード量を変えながら順次繰り返される。この３次元スキャンの場合、全体の撮像時間は比較的長くなるので、息止めは複数回に別けて実施される。
【０１０８】
画像再構成は、位相エンコード方向が左右ＲＬ方向に設定された３次元原データの１組で、また位相エンコード方向が上下ＨＦ方向に設定された３次元原データの別の１組で個別に実施される。両方の３次元の再構成データは画素毎に合成され、最終的な３次元のＭＲＡデータとなる。
【０１０９】
この３次元撮像によっても、前述した実施形態のものと同等に、最適化された心電同期タイミングによる描出能の向上を初めとして、血流の方向性の確保などの作用効果を得ることができる。
【０１１０】
第４の実施形態
本発明の第４の実施形態を図１４〜１５を参照して説明する。この例はマルチスライス・イメージングスキャンに関する。
【０１１１】
図１４は、ホスト計算機６およびシーケンサ５により指令される心電同期イメージングスキャンのシーケンスを例示している。このシーケンスでは、第３の実施形態と同様に、位相エンコード方向の制御、心電同期法としてのＥＣＧゲート法、および１回息止め法の各手法が採用されている。ＥＣＧゲート法による同期タイミングは、事前に実施されるＥＣＧ−ｐｒｅｐスキャンを通して最適化された遅延時間ＴDLで決められる。
【０１１２】
例えば、４枚のマルチスライスイメージングで腹部を撮像する場合、図１５に示す如く、各スライスに対応したスキャンＲＬ１，ＲＬ２，ＲＬ３，ＲＬ４，ＨＦ１，ＨＦ２，ＨＦ３，ＨＦ４，…の順序で例えば２次元ＦＡＳＥ法に基づきデータ収集される。第３の実施形態と同様に、スキャンＲＬは位相エンコード方向が左右ＲＬ方向、スキャンＨＦはそれが上下ＨＦ方向であることを示し、各回のスキャンにより各スライスの２次元原データが生成される。互いに位相エンコード方向が９０度異なる２フレームの再構成画像データが画素毎に合成され、各スライス面のＭＲＡ像データがつくられる。このため、高い血流方向の検出能が確保される。また、当然に、前述したＥＣＧゲート法や息止め法の効果もこのマルチスライスイメージングにおいて併せて発揮される。
【０１１３】
なお、このマルチスライスイメージングのスキャン順序は、ＲＬ１，ＨＦ１，ＲＬ２，ＨＦ２，…といった具合に任意の順に変更してもよい。
【０１１４】
なお、上述した各実施形態にあっては、イメージングスキャン時に位相エンコード方向を変えて複数回のスキャンを行うように設定していたが、本発明は必ずしもそのように位相エンコード方向を変える必要はない。予め定めた一定方向の位相エンコード方向のまま複数回のスキャンを行って複数組の画像データを生成し、その複数組の画像データをアベレージングして１組の画像データを得るように構成してもよい。
【０１１５】
第５の実施形態
本発明の第５の実施形態を図１６〜１８に基づき説明する。本実施形態は、複数種類の対象を撮像する場合に本発明を適用したものである。
【０１１６】
前述の各実施形態は、ＥＣＧ−ｐｒｅｐスキャンにより最適設定するＥＣＧゲート法（心電同期法）の同期タイミング（遅延時間ＴDL）は１つの量、すなわち１つの固定同期タイミングであった。
【０１１７】
撮像対象が１種類であるときは、この１つの同期タイミングでも間に合うが、撮像対象が図１６に示す如く、患者の大動脈ＡＲと肝臓門脈ＰＶであるとすると、前者はほぼ体軸方向に沿って走行し、後者は体軸方向に直交する左右方向に沿って走行する部分が多い。つまり、撮像対象としての血管の種類が異なると、その走行方向も異なり、ＥＣＧゲート法における最適な同期タイミングも異なることが一般的と想定される。そこで、本実施形態のＭＲＩ装置は、ＥＣＧゲート法の同期タイミングの数を撮像対象の種類、すなわち血管や組織の走行方向の違いに応じて複数個、設定することを特徴とする。
【０１１８】
具体的には、ホスト計算機６およびシーケンサ５は共働して図１７に示すように、心電同期イメージングスキャンに先立って、合計２回のＥＣＧ−ｐｒｅｐスキャンを順次実行するようになっている。最初のＥＣＧ−ｐｒｅｐスキャン＃１では、位相エンコード方向を例えば上下（体軸）ＨＦ方向に設定した状態で前述した図３、４に示すＥＣＧ−ｐｒｅｐスキャンが、一例として２次元ＦＡＳＥ法により実行される。この第１回目のＥＣＧ−ｐｒｅｐスキャンにより、図１６に示す例で言えば、大動脈ＡＲの走行方向の情報収集が重視された状態で、複数時相のスキャンが前述の如く実行される。この結果、大動脈ＡＲからのＭＲ信号の強度が最大になる最適遅延時間ＴDL＝α１が設定される。
【０１１９】
第２回目のＥＣＧ−ｐｒｅｐスキャンでは、位相エンコード方向を例えば左右ＲＬ方向に設定した状態で前述した図３、４に示すＥＣＧ−ｐｒｅｐスキャンが同じく２次元ＦＡＳＥ法により実行される。この第２回目のＥＣＧ−ｐｒｅｐスキャンにより、図１６に示す例で言えば、門脈ＰＶの走行方向の情報収集が重視された状態で、複数時相のスキャンが前述の如く実行される。この結果、門脈ＰＶからのＭＲ信号の強度が最大になる最適遅延時間ＴDL＝α２が設定される。
【０１２０】
その後、ホスト計算機６およびシーケンサ５は共働して、この２種類の遅延時間ＴDL＝α１，α２を使い、３次元ＦＡＳＥ法に基づく心電同期イメージングスキャンを実行する。このイメージングスキャンのシーケンス例を図１８に示す。同図に示すように、１回目の１つ目のスライスエンコード量ｓｅ１に対するスキャンのときには、門脈ＰＶの走行方向に合わせた遅延時間ＴDL＝α２に同期してスキャンされる。そして、これに呼応して、スキャンが位相エンコード方向＝左右ＲＬ方向で且つスライスエンコード量ｓｅ１の状態で３次元ＦＡＳＥ法に基づき実行される。さらに、２回目の１つ目のスライスエンコード量ｓｅ１に対するスキャンのときには、大動脈ＡＲの走行方向に合わせた遅延時間ＴDL＝α１に同期してスキャンされる。そして、これに呼応して、スキャンが位相エンコード方向＝上下ＨＦ方向で且つスライスエンコード量ｓｅ１の状態で３次元ＦＡＳＥ法に基づき実行される。
【０１２１】
以下、同様に、スライスエンコード量ｓｅを変えながら、遅延時間ＴDL＝α２に基づく位相エンコード方向＝左右ＲＬ方向の心電同期イメージングスキャンと、遅延時間ＴDL＝α１に基づく位相エンコード方向＝上下ＨＦ方向の心電同期イメージングスキャンとが交互に繰り返される。この一連のスキャンによって収集されたエコー信号は、第１の実施形態と同様に処理され、表示される。
【０１２２】
このように、個別のＥＣＧ−ｐｒｅｐスキャンにより、ＥＣＧゲート法のための同期タイミング（遅延時間）が位相エンコード方向を２種類の撮像対象の走行方向に合わせて個別に設定され、この２つの量の同期タイミングそれぞれに基づき２種類の撮像対象（大動脈や門脈）の走行方向に合わせた位相エンコード方向で心電同期イメージングスキャンが行われ、ＭＲ画像が生成される。
【０１２３】
このため、位相エンコード方向を２種類の撮像対象それぞれに合わせて撮像するときの前述した効果は勿論のこと、ＥＣＧゲート法の同期タイミング自体を、患者別に、撮像対象別に、かつ、使用するパルスシーケンス別に対応して設定している。したがって、例えば血流速度の如何を問わず、目的とする血管からの信号値が最も大きい状態での心電同期イメージングスキャンが行われるので、２種類の撮像対象が確実に捕捉される。つまり、全部の撮像対象の走行情報が十分に確保され、高描出能で、高いＳ／ＮのＭＲ画像が提供される。
【０１２４】
なお、上述の説明は撮像対象が大動脈と門脈の２種類である場合であったが、３種類以上の撮像対象であっても同様で、その種類数だけのＥＣＧ−ｐｒｅｐスキャンを実施して撮像対象別の同期タイミングを設定すればよい。また、この複数個の同期タイミングを使用する心電同期イメージングスキャンは、図１８の３次元ＦＡＳＥ法による３次元撮像に限定されることなく、図９に示した２次元ＦＡＳＥ法によるシングルスライス撮像や、図１４に示した２次元ＦＡＳＥ法によるマルチスライス撮像であってもよい。さらに、それらの撮像に使用するパルスシーケンスもＦＡＳＥ法に限定されることなく、高速ＳＥ法であっても、またＦＥ系のパルスシーケンスであってもよい。
【０１２５】
第６の実施形態
本発明の第６の実施形態を図１９〜図２３に基づき説明する。
【０１２６】
この実施形態に係るＭＲＩ装置は、パルスシーケンスとして心臓系の撮像に好適なセグメンテッド（segmented ）ＦＦＥ法（以下、ｓｅｇＦＦＥ法と呼ぶ）を採用したことを特徴とする。一例として、ＥＣＧ−ｐｒｅｐスキャンを２次元のｓｅｇＦＦＥ法で実施し、その後の心電同期イメージングスキャンを３次元のｓｅｇＦＦＥ法で実施する場合を説明する。このように、ＥＣＧ−ｐｒｅｐスキャンの次元を減らすことで、心電同期タイミングの測定時間を短縮させることができる。
【０１２７】
このｓｅｇＦＦＥ法を採用したシーケンスの具体例を図１９〜２１に示す。図１９にはＥＣＧ−ｐｒｅｐスキャンの概要を、図２０にはＥＣＧ−ｐｒｅｐスキャンの２次元ｓｅｇＦＦＥ法に依るパルスシーケンスの一例をそれぞれ示す一方で、図２１には心電同期イメージングスキャンの概要を、図２２には心電同期イメージングスキャンの３次元ｓｅｇＦＦＥ法に基づくパルスシーケンスの一例をそれぞれ示す。このＥＣＧ−ｐｒｅｐスキャンおよびイメージングスキャンには図示していないが、息止め法が併用される。
【０１２８】
ＥＣＧ−ｐｒｅｐスキャンは「シングルスライス・マルチフェーズ」と呼ばれる方式を採用している。ホスト計算機６はシーケンサ５に対してこの方式に基づく２次元ｓｅｇＦＦＥ法のパルスシーケンスを指令する。
【０１２９】
この「シングルスライス・マルチフェーズ」方式によれば、スライス用傾斜磁場ＧS およびＲＦ周波数で決まるシングルスライスに対して複数の時相のエコーデータを一度に収集することができる。このため、そのスライスに撮像目的の血管が入るようにそのスライス厚さが決められる。
【０１３０】
これを具体的に説明すると、セグメントと呼ぶ一塊の連続データが得られるＥＣＧ信号のＲ−Ｒ波間において、Ｒ波ピーク値の出現時刻から遅延時間ＴDL＝α１が経過した時刻から、図１９、２０のセグメント１のフェーズ１で示す如く、複数組のＲＦ励起およびＦＥ法によるエコー収集が繰り返される（ここでは４個のエコー信号収集）。これにより収集された複数個（ここでは４個）のエコー信号は受信処理を経て、演算ユニット１０に送られる。演算ユニット１０には、位相エンコード方向を複数個（ここでは４個）に分割してｋ空間が複数個（ここでは５個）形成されており、フェーズ１で収集された複数個のエコー信号はその位相エンコード量に応じて最初のｋ空間ＫＳ１の各分割領域の最初のラインに配置される（図１９参照）。
【０１３１】
さらに、Ｒ波ピーク値の出現時刻から遅延時間ＴDL＝α２（＞α１）が経過した時刻から、セグメント１のフェーズ２で示す如く、複数組のＲＦ励起およびＦＥ法によるエコー収集が繰り返される。この結果得られたエコー信号は、次のｋ空間ＫＳ２の各分割領域の最初のラインに配置される。遅延時間ＴDL＝α３（＞α２），α４（＞α３），α５（＞α４）のそれぞれについても同様にして４個のエコー信号が収集され、３番目、４番目、および５番目のｋ空間ＫＳ３，ＫＳ４，およびＫＳ５の各分割領域の最初のラインに配置される。この結果、５個のｋ空間それぞれの各分割領域の最初のラインにエコーデータが配置される。
【０１３２】
次いで、次のＲ−Ｒ波間であるセグメント２に対しても同様のエコー収集および配置が実行される。ただし、このときの収集データは各ｋ空間の各分割領域の次のラインに配置される。以下、同様にセグメント３、４、および５について実行される。
【０１３３】
したがって、このＥＣＧ−ｐｒｅｐスキャンが終了すると、５個のｋ空間全部のデータ配置が完了している。演算ユニット１０はこれら５個のｋ空間のデータを２次元フーリエ変換して、５枚の実空間画像を再構成する。つまり、遅延時間ＴDL＝α１，α２，α３，α４，α５の時相の準備用画像を１回のスキャンで一度に得ることができる。そこで、オペレータは、この５枚の準備用画像を例えば目視観察して、撮像部位が最も明瞭に表示されている画像、すなわち最適な遅延時間ＴDLの値を特定する。
【０１３４】
次いで、適宜なタイミングで、ホスト計算機６はシーケンサ５に対して３次元ｓｅｇＦＦＥ法の心電同期イメージングスキャンを図２１、２２に示す如く指令する。このイメージングスキャンで用いられている最適遅延時間ＴDL＝αｍは、上述のＥＣＧ−ｐｒｅｐスキャンで設定された値である。
【０１３５】
このイメージングスキャンの各セグメントに対するパルス列は、最初に印加するＭＴ（magnetization transfer）効果を与えるＭＴパルスＰｍｔと、その次に印加する脂肪抑制用の化学選択パルスＰchess と、その次に各傾斜磁場方向に印加するディフェーズ用のスポイラパルスＳＰｓ，ＳＰｒ，ＳＰｅとを準備用パルス列として含む。この準備用パルス列の後には、フィールドエコーを収集するためのデータ収集パルス列Ｐａｃｑを配置してある。このイメージングスキャンは３次元スキャンであるので、位相エンコード用傾斜磁場ＧE1の他に、スライス用傾斜磁場ＧE2が印加される。
【０１３６】
このセグメント化された複数個のエコー信号は、そのスライスエンコード量および位相エンコード量に応じて演算ユニット１０の３次元ｋ空間に配置される。この配置データはその後、３次元フーリエ変換により実空間データに再構成され、さらに例えばＭＩＰ処理により２次元画像に変換される。
【０１３７】
このように本実施形態によれば、前述した実施形態のように位相エンコード方向を空間的に変えて複数回スキャンするスキャン法を採用していないが、ｓｅｇＦＦＥ法を用いて心臓系を確実に撮像することができる。とくに、このときの心電同期タイミングを事前に最適化しているので、心臓系の速い血流をも確実に捕捉することができる。
【０１３８】
この心電同期タイミングは上述のように「シングルスライス・マルチフェーズ」方式で測定されるので、１回のＥＣＧ−ｐｒｅｐスキャンで複数の時相の画像を一度に得ることができる。つまり、複数回のＥＣＧ−ｐｒｅｐスキャンを行う必要がなく、全体の撮像時間を短縮でき、患者スループットを向上させる。
【０１３９】
なお、このｓｅｇＦＦＥ法を用いたＥＣＧ−ｐｒｅｐスキャンは上述の「シングルスライス・マルチフェーズ」方式に限定されるものではなく、例えば、図２３に示す如く、「マルチスライス・シングルフェーズ」方式を簡便的な手法として採用することもできる（ここでのシングルフェーズは各スライスに１つのフェーズを意味する）。同図に示す如く、各セグメントにおけるスライス１の複数個のエコー信号を集めて１枚のスライス＃１のｋ空間ＫＳ１を埋める。また、各セグメントのスライス２の複数個のエコー信号を集めて別の１枚のスライス＃２のｋ空間ＫＳ２を埋める。同様に、各セグメントのスライス３，４，５の複数個のエコー信号をそれぞれ集めて別の１枚のスライス＃３，＃４，＃５のｋ空間ＫＳ３，ＫＳ４，ＫＳ５をそれぞれ埋める。これにより、遅延時間ＴDL＝α１、α２、α３、α４、α５で決まる、収集時相が相互に異なる複数枚のスライス＃１〜＃５の画像が１回のｓｅｇＦＦＥ法のスキャンで得られる。
【０１４０】
この複数枚のスライス＃１〜＃５は互いに近接しているので、撮像部位の心電同期タイミングを共にほぼ正確に反映した１枚のスライス（画像）であると見做しても差支えない場合も多い。このような場合、この複数枚の画像を１枚の画像として扱い、これらの画像から最適な遅延時間を設定すればよい。とくに、この簡便な「マルチスライス・シングルフェーズ」方式を使用できる場合、ＳＡＲ（ＲＦ被爆）の点で「シングルスライス・マルチフェーズ」方式よりも有利である。
【０１４１】
なお、このｓｅｇＦＦＥ法と同様の手法をＥＰＩ法にも適用できる。つまり、ＥＣＧ−ｐｒｅｐスキャンは２次元のＥＰＩ法で実施し、これにより最適化した遅延時間を用いて３次元のＥＰＩ法に基づくイメージングスキャンを実行するものである。
【０１４２】
第７の実施形態
本発明の第７の実施形態を図２４〜図２６に基づき説明する。
【０１４３】
この実施形態のＭＲＩ装置は、ＥＣＧ−ｐｒｅｐスキャンにより得られた複数時相の画像データからの最適遅延時間の選択処理およびその遅延時間のイメージングスキャンへの反映処理を省力化することに特徴を有する。
【０１４４】
この特徴を達成するため、ホスト計算機６は図２４に示す一連の処理を行う。前述した各実施形態のように、ＥＣＧ−ｐｒｅｐスキャンにより複数時相（遅延時間）の準備用画像が得られると、ホスト計算機６は図２４の処理に移行し、その複数の準備用画像ＩＭ１〜ＩＭ５を図２５に示す如く表示する（ステップＳ４１）。この準備用画像ＩＭ１〜ＩＭ５のそれぞれには遅延時間ＴDLの値も重畳表示される（ステップＳ４２）。
【０１４５】
この表示が済むと、ホスト計算機６はラインＲＯＩを図２５に示す如く、準備用画像ＩＭ１〜ＩＭ５の初期位置に重畳表示する（ステップＳ４３）。次いで、入力器１３からの入力信号を読み込み、オペレータの指定に応じてＲＯＩ位置を調整し、ＲＯＩ位置がそれでＯＫか否かを判定する（ステップＳ４４，Ｓ４５）。これにより、オペレータとの間で対話的にオペレータが望む例えば血管上の位置にラインＲＯＩが設定される。
【０１４６】
このＲＯＩ設定が終わると、ホスト計算機６は各準備用画像ＩＭ上の指定ライン位置の画素について信号強度分布ＤＳを演算し、その分布ＤＳを各画像上に重畳表示する（ステップＳ４６、Ｓ４７）。この表示の一例を図２６に示す。
【０１４７】
ホスト計算機６は信号強度（画素値）分布の例えば最高値を呈する画像（分布）を特定する（ステップＳ４８）。この「最高値」は、ＲＯＩ位置の信号強度が最も大きい画像（分布）を特定するための１つの指標であり、これ以外の指標を用いてもよい。なお、ラインＲＯＩの代わりに矩形状のＲＯＩを用い、信号強度分布の代わりにヒストグラムを用いてもよい。
【０１４８】
次いで、ホスト計算機６は特定された準備用画像、すなわちその画像の時相（遅延時間）ＴDLを最適な心電同期タイミングとして確定し、その値を内部メモリに記憶する（ステップＳ４９）。この記憶された遅延時間ＴDLは、イメージングスキャン時にそのメモリから自動的に読み出される。すなわち、前述した図７のステップＳ２０の処理に対応するステップＳ５０の処理において、手動により指定されるのではなく、既に記憶していた最適遅延時間ＴDLが自動的に読み出される。
【０１４９】
これにより、ＥＣＧ−ｐｒｅｐスキャンを撮像時間の短い２次元スキャンのＦＡＳＥ法、ＥＰＩ法、ｓｅｇＦＦＥ法で行い、最適な心電同期タイミングをオペレータとの間でインターラクティブに確定し、その心電同期タイミングをイメージングスキャンに自動的に反映させることができる。
【０１５０】
したがって、オペレータがＥＣＧ−ｐｒｅｐスキャンで得た複数の準備用画像を判定するときの労力を軽減し、また判定誤差を減らして、より正確な心電同期タイミングを求めることができる。これにより、心電同期イメージングスキャンにおける血流の捕捉能力を高めることができる。また、ＥＣＧ−ｐｒｅｐスキャンに基づく心電同期タイミングの決定結果を自動的に心電同期イメージングスキャンに反映させるので、この点からも操作上の労力が著しく軽減される。
【０１５１】
なお、上記実施形態では複数枚の準備用画像の内の最も適切と思われる画像を信号強度分布などの演算結果から選択的に決め、その画像に割り当てられている遅延時間ＴDLを適切な同期タイミングとして決するように構成したが、本発明にかかる同期タイミングの決定方法はこれに限定されるものではない。要するに、複数枚の準備用画像のデータを利用して適切を思われる同期タイミングを選択または決定するものであればよい。
【０１５２】
例えば、上述の実施形態で複数枚の準備用画像から２枚以上の準備用画像を適切に選択し、これらの選択画像に割り当てられている複数の遅延時間値からカーブフィッティング法などの周知の手法に基づき、最適な遅延時間を更に演算して求めるようにしてもよい。
【０１５３】
例えば、準備用画像を表示することなく、信号強度分布やヒストグラムなどを画像データから自動的に演算し、その演算結果を所定のアルゴリズムで自動的に判定し、その判定結果から同期タイミングを自動的に決めるようにしてもよい。この同期タイミングは前述したように自動的にＥＣＧ−ゲートのイメージングスキャンに反映させることで、ＥＣＧ−ｐｒｅｐスキャンとイメージングスキャンの間で同期タイミング設定に関するオペレータの手間を全く不要にすることができる。
【０１５４】
ところで、上述した各実施形態およびその変形例では、ＭＲアンギオグラフィ（ＭＲＡ）を目的としていたが、撮像対象は血管のみに限定されず、繊維状に走行する組織等、任意の対象のものであってよい。とくに、Ｔ₂ 時間が短めなものであれば、本発明に係るＥＣＧ−ｐｒｅｐスキャンおよびその後のイメージングスキャンを好適に実施できる。
【０１５５】
【発明の効果】
本発明のＭＲＩ装置によれば、心電同期法を併用して撮像する場合、１回のＲＦ励起（１ショット）に伴う撮像時間の長短に関わらず、複数心拍それぞれに対する準備用ＭＲスキャンの実行を通して、心電同期法の同期タイミング（時相）が予め適切に決定される。このため、イメージング用スキャンとして心電同期スキャンを行うときの同期タイミングが診断部位を流れる血流などの撮像対象にとって適切な状態となり、撮像対象から発生するエコー信号の強度が最も高くなる。この結果、いわゆるフローボイド現象などに因ってエコー信号の強度が相対的に低下または殆ど零となる状態を確実に回避でき、撮像対象自体の描出およびその方向性の情報量を向上させた視認性の良い高描出能のＭＲ像を安定して提供することができる。
【０１５６】
とくに、例えばＴ₂ 時間がＴ₂ ＝１００〜２００msと短めの血液を撮像するときに、その効果が著しく発揮される。同時に、準備用スキャンに息止め法を併用することで、体動アーチファクトの少ない、高品質のＭＲ画像を準備でき、これにより心電同期の同期タイミングの設定精度を更に向上させることができる。
【０１５７】
また、本発明によれば、心電同期タイミングの適切化を図る際、少なくとも準備用ＭＲスキャンにおいて、その位相エンコード方向を撮像対象の走行方向に合わせるため、例えば横緩和時間が短めの組織や血流の走行方向をも確実に描出でき、その走行情報を豊富に提供することができる。
【０１５８】
さらに、本発明によれば、心電同期タイミングの適切化の操作をオペレータとのインターラクティブな操作や自動演算により極力、省力化および自動化する一方で、確定した最適な心電同期タイミングをイメージング用ＭＲスキャンに自動的に反映するようにしているので、オペレータの操作上の手間や負荷を軽減でき、さらに患者スループット向上にも寄与できる。
【図面の簡単な説明】
【図１】本発明の実施形態に係るＭＲＩ装置の構成の一例を示す機能ブロック図。
【図２】実施形態におけるＥＣＧ−ｐｒｅｐスキャンと心電同期イメージングスキャンの時間の前後関係を説明する図。
【図３】コントローラが実行するＥＣＧ−ｐｒｅｐスキャンの手順を例示する概略フローチャート。
【図４】ＥＣＧ−ｐｒｅｐスキャンの一例を示すタイミングチャート。
【図５】ＥＣＧ−ｐｒｅｐスキャンにより得られた、遅延時間をダイナミックに変化させたときの肺野のＭＲＡ像を模式的に写生した図。
【図６】位相エンコード方向の信号値の広がりを説明する図。
【図７】実施形態にてホスト計算機が実行するイメージングスキャンの処理例を示す概略フローチャート。
【図８】同実施形態にてシーケンサが実行するスキャン制御の処理例を示す概略フローチャート。
【図９】同実施形態におけるイメージングキャンのスキャン順と画像合成の関係を模式的に説明する図。
【図１０】単独スキャン時の異なる位相エンコード方向における見掛け上の信号値の広がりを反映した画素列と画像合成時のそれを例示する模式図。
【図１１】本発明の第２の実施形態に係るイメージングスキャンのスキャン順と画像合成の関係を模式的に説明する図。
【図１２】本発明の第３の実施形態に係る３次元イメージングスキャンのスキャン順と画像合成の関係を模式的に説明する図。
【図１３】３次元スキャンに係るボリューム領域と傾斜磁場方向の設定の関係を説明する図。
【図１４】本発明の第４の実施形態に係るマルチスライス・スキャンのスキャン順と画像合成の関係を模式的に説明する図。
【図１５】マルチスライス・スキャンに係るスライス面の位置関係を説明する図。
【図１６】本発明の第５の実施形態に係るＥＣＧ−ｐｒｅｐスキャンの撮像対象別の位相エンコード方向の設定を説明する図。
【図１７】２段階のＥＣＧ−ｐｒｅｐスキャンの時間関係を説明する図。
【図１８】２つの値の同期タイミングを用いた心電同期イメージングスキャンを説明する部分的タイムチャート。
【図１９】本発明の第６の実施形態に係るＥＣＧ−ｐｒｅｐスキャンのｓｅｇ．ＦＦＥ法のシーケンス、収集時相、およびエコーデータ配置の関係を説明する図。
【図２０】２次元ｓｅｇ．ＦＦＥ法のパルス列の一例を説明するシーケンス。
【図２１】第６の実施形態に係るイメージングスキャンのｓｅｇ．ＦＦＥ法のシーケンス、収集時相、およびエコーデータ配置の関係を説明する図。
【図２２】３次元ｓｅｇ．ＦＦＥ法のパルス列の一例を説明するシーケンス。
【図２３】第６の実施形態の変形例に係るＥＣＧ−ｐｒｅｐスキャンのｓｅｇ．ＦＦＥ法のシーケンス、収集時相、およびエコーデータ配置の関係を説明する図。
【図２４】本発明の第７の実施形態に係る最適同期タイミングの確定の自動化処理を示す概略フローチャート。
【図２５】最適同期タイミングの確定処理の一過程を示す準備用画像の表示状態図。
【図２６】最適同期タイミングの確定処理の別の一過程を示す準備用画像の表示状態図。
【符号の説明】
１ 磁石
２ 静磁場電源
３ 傾斜磁場コイルユニット
４ 傾斜磁場電源
５ シーケンサ
６ コントローラ
７ ＲＦコイル
８Ｔ 送信器
８Ｒ 受信器
１０ 演算ユニット
１１ 記憶ユニット
１２ 表示器
１３ 入力器
１６ 音声発生器
１７ ＥＣＧセンサ
１８ ＥＣＧユニット
１９ 音声発生器[0001]
BACKGROUND OF THE INVENTION
  The present invention relates to magnetic resonance imaging for imaging the inside of a subject based on the magnetic resonance phenomenon of the subject. In particular, imaging based on the ECG synchronization method is performed using a signal representing the cardiac time phase of the subject.For MRI (magnetic resonance imaging) equipmentRelated.
[0002]
[Prior art]
Magnetic resonance imaging (MRI) is an imaging method in which a nuclear spin of a subject placed in a static magnetic field is magnetically excited with a high-frequency signal of its Larmor frequency, and an image is reconstructed from an MR signal generated by the excitation. Is the law.
[0003]
For example, when imaging blood vessels in the lung field, blood vessels in the liver (portal veins), etc. by this magnetic resonance imaging, increasing the signal value of the blood vessel image to improve S / N, reducing artifacts due to body movement, etc. There are various demands.
[0004]
Under such demand, various types of imaging methods are used as magnetic resonance imaging. One of them is the Echo Planar Imaging (EPI) method, which is an ultra-high-speed imaging method. This imaging method is a pulse sequence that collects MR signals at high speed by inverting the readout gradient magnetic field at high speed after one RF excitation. Is used. This EPI imaging is advantageous in that the time required for data collection is short, so that there is little room for artifacts due to body movement. Another imaging method is a high-speed SE (high-speed spin echo) method or an imaging method using the high-speed SE (high-speed spin echo) method, in which the acquisition time associated with one RF excitation (that is, one shot) is compared with one heartbeat. Long imaging method (for example, the imaging time is about 300 ms. Depending on the size of the imaging matrix, it is about 600 ms). This imaging method based on the high-speed SE method has a relatively long imaging time compared to the EPI method, but on the other hand, it has advantages such as strong susceptibility and less distortion of the form. Yes. For such imaging, an ECG gate method as an electrocardiographic synchronization method that has been generally known can be used in combination.
[0005]
In addition, when the imaging target is a heart system, imaging using an FE pulse sequence with a short imaging time associated with one RF excitation is also performed. In particular, in recent years, the segmented FFE method has been frequently used. Also in the case of this segmented FFE method, a method of matching the collection time phase for each segment by using the ECG gate method is preferable.
[0006]
[Problems to be solved by the invention]
However, when imaging using the ECG gate method in combination with the various pulse sequences described above, the following unsolved problems have been left unattended.
[0007]
That is, there is a problem of optimization of timing of ECG synchronization. With regard to the electrocardiographic synchronization, there has been little research on the optimization of the synchronization timing. There should be a more appropriate synchronization timing depending on the individual difference of the subject and the diagnosis site (for example, whether it is near or far from the heart) and the type of pulse sequence to be used. No specific research or proposal has been made regarding the synchronization timing. For this reason, if the ECG gate method is to be used together in the above-described imaging method, the operator gives a synchronization timing that seems appropriate based on experience or trial and error. However, in that case, due to the flow void phenomenon, the signal value becomes low, and the examiner may cause a situation in which the MRA image that accurately captures the intended blood flow cannot be obtained. is there. In other words, there is no guarantee that the ECG gate method is fully utilized and stable imaging is performed in which the object is reliably captured.
[0008]
Under such circumstances, for example, when it is desired to image a relatively high-speed aortic flow of 2 m / sec, if the synchronization timing of ECG synchronization is not set appropriately, the above-described flow void phenomenon or the like is caused and it is ensured. There is a high possibility that it cannot be imaged.
[0009]
The present invention has been made to overcome the current state of the prior art, and one of its purposes is to optimally set the ECG synchronization timing in advance when imaging using the ECG synchronization method in advance. This is to provide a stable and high-definition MR image.
[0010]
In addition, another object of the present invention is to make it possible to reliably depict the tissue and blood flow direction in which the lateral relaxation time is short, and to enrich the travel information when optimizing the ECG synchronization timing. That is.
[0011]
Furthermore, another object of the present invention is to reduce the labor and load on the operation of the operator by optimizing the operation of optimizing the ECG synchronization timing as much as possible.
[0012]
[Means for Solving the Problems]
  In order to achieve the above object, according to the first aspect of the MRI apparatus of the present invention, for a desired region of a subject,Collect signals representing the cardiac phase of the subjectIn an MRI apparatus that performs MR scanning in synchronization with a signal representing the cardiac time phase of the subject collected by the signal collecting means, different synchronization times based on a plurality of reference waves of the signal representing the cardiac time phase of the subject For the region of the subjectFor every multiple heartbeatsPreparation scan means for executing MR scan for preparation a plurality of times and collecting MR signals respectively, and for preparing a plurality of sheets corresponding to the plurality of different synchronization times from the MR signalMRA imagePreparation image generating means for generating a plurality of sheets for preparationMRA imageFor specific preparationMRA imageInformation about the synchronization time collectedMRAThe preparation information reflecting means to be reflected in the MR scan for imaging, and the information on the synchronization time reflected by the preparation information reflecting meansMRAAnd a control means for executing an MR scan for imaging.
[0013]
  For example, the preparation information reflecting means is for preparing the plurality of sheets.MRA imageAnd display means for displaying the multiple preparations displayedA selection means for selecting a desired preparation MRA image from among the MRA images, and a preparation MR scan collected for obtaining a preparation MRA image selected by the selection means.Information about sync timeMRAReflecting means for reflecting in the MR scan for imaging. The preparation information reflecting means is for preparing the plurality of sheets.MRA imageAnd display means for displaying the multiple preparations displayedMRA imageManual designating means for manually designating a desired position, and for the preparation at the designated positionMRA imageCalculating means for automatically calculating information on the strength of the data, and for the plurality of preparations based on the calculation result of the calculating meansMRA imageFor the desired preparationMRA imageThe selection means to automatically select and for this selected preparationMRA imageAutomatic reflection means for automatically reflecting the synchronization time included in the imaging MR scan as the synchronization time for the electrocardiogram synchronization method may be provided.
[0014]
  According to the second aspect of the MRI apparatus of the present invention,Collect signals representing the cardiac phase of the subjectWith respect to the region including the imaging target of the subject at different synchronization times based on a plurality of reference waves of the signal representing the cardiac time phase of the subject collected by the signal collecting meansFor every multiple heartbeatsPreparation scan means for executing MR scans for preparation a plurality of times to collect MR signals, and a plurality of preparations corresponding to the plurality of different synchronization times from the MR signalsMRA imagePreparing image generating means for generating the plurality of sheets for preparingMRA imageBased on informationMRASynchronization time specifying means for specifying the synchronization time of the MR scan for imaging, and the region of the subject in synchronization with the specified synchronization timeMRAAnd imaging scanning means for executing an imaging MR scan.
[0015]
In the configuration according to the second aspect, for example, the signal collecting means is means for collecting an ECG signal of the subject as a signal representing the cardiac time phase. In addition, for example, the preparation scanning unit includes a changing unit that changes a delay time from an R wave as a reference wave included in the ECG signal into a plurality of time values, and an elapsed time of each of the changed delay times. Scan start means for starting the preparatory MR scan as the synchronization time. As an example, the imaging target of the subject to be scanned by the preparation scanning unit is a tissue or blood flow in which the transverse relaxation time of the nuclear spin is short.
[0016]
  Particularly preferably, the MR scan for preparation and theMRABoth MR scans for imaging use the same type of pulse sequence. As an example, this pulse sequence is a pulse sequence of a high-speed SE method or a pulse sequence of the FASE method using this high-speed SE method. In this pulse sequence, for example, the scan time per RF excitation is about 200 msec or more. In addition, this pulse sequence is a sequence that depends on the Fourier transform method for reconstructing a real space image by arranging the MR signal in a frequency space and performing a Fourier transform, for example, and the frequency space is in the phase encoding direction. A sequence in which the MR signals are arranged by dividing each predetermined amount.
[0017]
As another example, the pulse sequence is a pulse sequence of a segmented fast FE method. This segmented high-speed FE method pulse sequence, for example, collects a plurality of echo signals at each of a plurality of different times between the R waves, and a plurality of pulse signals corresponding to a single slice passing through the region of the subject. Each of the two-dimensional k-spaces has a pulse train that arranges the echo signals at the same time.
[0018]
As another example, the pulse sequence is an EPI pulse sequence.
[0019]
  From another viewpoint, the preparatory MR scan uses a two-dimensional scan pulse sequence, andMRAThe imaging MR scan may use a three-dimensional scan pulse sequence.
[0020]
According to the above configuration, the electrocardiographic synchronization timing when imaging using the electrocardiographic synchronization method is set appropriately or optimally in advance, and a stable and high rendering capability MR image is provided.
[0021]
As an example, even if the imaging time associated with one RF excitation is based on a pulse sequence that is relatively long compared to one heartbeat, preparation is performed while dynamically changing the delay time for each of a plurality of heartbeats. MR scan is performed a plurality of times. Specifically, various delay times from each of the plurality of R waves are adjusted. By appropriately changing the magnitude of this delay time, a turbulent flow time zone immediately after the appearance of the R wave can be avoided, and a plurality of preparatory MRs adapted to a relatively stable flow time zone. A scan can be performed. Therefore, a plurality of reconstructed images having relatively good contrast and different delay times can be obtained. From this image, the image in which the imaging target such as the aorta is best depicted can be selected, and the delay time when collecting the image data can be determined as an appropriate or optimal synchronization timing.
[0022]
Therefore, when performing MR scan for electrocardiographic imaging, the imaging target is, for example, the lateral relaxation time T₂Even if the blood is shorter, an appropriate or optimal synchronization timing for the blood is set in advance. In other words, even if there are individual differences in the subject itself, differences in the diagnostic region, and differences in the imaging target to be diagnosed, the synchronization timing is determined in terms of the MR signal value for each subject, diagnostic region, and imaging target. Is always set to a desired appropriate timing. For this reason, it is possible to reduce or eliminate a situation such as a decrease in signal value due to a flow void phenomenon or the like, and to increase the intensity of an echo signal from an imaging target such as a blood flow. Therefore, it is possible to obtain a high-quality MR image with high rendering performance that reliably captures the imaging target.
[0023]
As another example, the imaging time associated with one excitation is relatively short. Even when the heart is imaged using the electrocardiographic synchronization method in combination with the FFE method, the electrocardiographic synchronization timing is appropriately set in advance, so that an MR image with high imaging ability that reliably captures the imaging target is provided. The
[0024]
  Furthermore, in the MRI apparatus of the present invention, in the configuration of the second aspect, the preparation scanning means includes a plurality of types of blood vessels to be imaged.In a plurality of scans executed by changing the phase encoding direction, the phase encoding direction is substantially matched with the traveling direction of the blood vessel that branches.Execution means for executing the preparation MR scan for each imaging target; and the preparation image generating means for preparing the plurality of sheets from the MR signals collected in the preparation MR scan for each imaging target.MRA imageThe synchronization time specifying means is for preparing the plurality of sheets for each imaging target.MRA imageThe desired synchronization time based on the information of the imaging, and the scanning means for imaging is a plurality of types of the imaging targetBlood vesselsIn the direction of travelApproximate matchSynchronized with the desired synchronization time specified for each imaging target in the phase encoding direction.MRAIt is possible to have execution means for executing the MR scan for imaging for each imaging target.
[0025]
The plurality of types of imaging objects are, for example, a plurality of types of bloodstreams that travel in spatially different directions within the region of the subject. Further, for example, the imaging scanning unit includes a collecting unit that collects echo signals collected in the MR scan for imaging for each imaging target, and a generating unit that generates image data of the region from the echo signals for each imaging target. Is provided. Further, for example, the generating means includes means for reconstructing the echo signal into real space image data for each imaging target, and means for synthesizing the reconstruction data for each imaging target for each pixel.
[0026]
  In the configuration of the second aspect described above, the preparation scanning unit includes a unit that sets a phase encoding direction used for the preparation MR scan in accordance with a traveling direction of the imaging target.Can also.
[0027]
With the above configuration, since the blur in the phase encoding direction of the pixel value in the preparation image matches the traveling direction of the imaging target such as blood flow, the traveling direction is enhanced and the MR image is enhanced, and the direction rendering capability of the imaging target is improved. improves. Therefore, the accuracy when selecting and setting the electrocardiogram synchronization timing using this preparation image can be remarkably improved. In addition, in the imaging scanning means, the phase encoding direction is matched with the traveling direction of the imaging target, so the direction rendering ability of the finally obtained MR image is improved.
[0028]
Furthermore, the MRI apparatus of the present invention, in the configuration of the second aspect, has a breath holding command for instructing the subject to perform breath holding while the preparation scanning means is executing the preparation MR scan. Means can also be provided. Since the body movement of the subject during the preparatory MR scan is reduced by using the breath holding method in this way, an MR image in which the occurrence of artifacts due to this body movement is suppressed can be obtained. Therefore, the accuracy when selecting and setting the electrocardiogram synchronization timing using this MR image can be remarkably improved.
[0029]
Furthermore, in the configuration of the second aspect, the MRI apparatus of the present invention is characterized in that the synchronization time specifying means includes interface means for presenting the plurality of preparation images to the operator and receiving designation of a part desired by the operator. And determining means for determining the desired synchronization time from data relating to the intensity of the echo signal of the region of interest. As an example, the synchronization time specifying unit may further include a reflection unit that automatically reflects the determined desired synchronization time in the imaging MR scan sequence. As another example, the interface means may include a display means for superimposing and displaying information representing each of the plurality of different synchronization times on each of the plurality of preparation images. As yet another example, the interface means includes ROI designating means for designating a site desired by the operator by ROI, while the determining means is a pixel corresponding to the signal intensity distribution of the ROI portion. Means for calculating from the value and means for determining the desired synchronization time from the signal intensity distribution by a predetermined algorithm may be provided.
[0030]
As a result, the operation for setting the electrocardiogram synchronization timing is made labor-saving or automated as much as possible, so that the labor and load on the operation of the operator are reduced.
[0031]
  Furthermore, according to the MRI apparatus of the present invention, in the MRI apparatus for performing MR scan in synchronization with a signal representing the cardiac time phase of the subject collected by the signal collecting means on a desired region of the subject. Preparation for collecting MR signals by executing multiple preparatory MR scans for the region of the subject at different synchronization times based on a plurality of reference waves of a signal representing the cardiac phase of the specimen. Scanning means for preparing a plurality of sheets corresponding to the plurality of different synchronization times from the MR signalMRA imagePreparation image generating means for generating a plurality of sheets for preparationMRA imagePreparation information reflecting means for reflecting the information on the MR scan for imaging,Prepared for this preparation information reflection meansFor preparing the plurality of sheetsMRA imageAnd display means for displaying the multiple preparations displayedMRA imageFor the desired preparationMRA imageSelection means for selecting and for the preparation selected by this selection meansMRA imageBased on the information about the delay time of the preparatory MR scan collected to obtainMRAAnd a control means for executing an MR scan for imaging.
[0032]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the accompanying drawings.
[0033]
First embodiment
A first embodiment will be described with reference to FIGS.
[0034]
A schematic configuration of an MRI (magnetic resonance imaging) apparatus according to this embodiment is shown in FIG.
[0035]
This MRI apparatus includes a bed part on which a patient P as a subject is placed, a static magnetic field generation part for generating a static magnetic field, a gradient magnetic field generation part for adding position information to the static magnetic field, and an RF (high frequency) signal. A transmission / reception unit for transmitting and receiving, a control / calculation unit responsible for overall system control and image reconstruction, an electrocardiogram measurement unit for measuring an ECG (electrocardiogram) signal as a signal representing the cardiac phase of the patient P, and a patient P A breath-hold command unit that commands breath-holding is functionally provided.
[0036]
The static magnetic field generation unit includes, for example, a superconducting magnet 1 and a static magnetic field power supply 2 that supplies current to the magnet 1, and a longitudinal axis of a cylindrical opening (diagnostic space) into which the subject P is loosely inserted. Static magnetic field H in the direction (Z-axis direction)₀Is generated. In addition, the shim coil 14 is provided in this magnet part. The shim coil 14 is supplied with a current for homogenizing a static magnetic field from a shim coil power supply 15 under the control of a host computer to be described later. The couch portion can removably insert the top plate on which the subject P is placed into the opening of the magnet 1.
[0037]
The gradient magnetic field generator includes a gradient magnetic field coil unit 3 incorporated in the magnet 1. The gradient coil unit 3 includes three sets (types) of x, y, and z coils 3x to 3z for generating gradient magnetic fields in the X, Y, and Z axis directions orthogonal to each other. The gradient magnetic field unit further includes a gradient magnetic field power supply 4 that supplies current to the x, y, z coils 3x to 3z. This gradient magnetic field power supply 4 supplies a pulse current for generating a gradient magnetic field to the x, y, z coils 3x to 3z under the control of a sequencer described later.
[0038]
By controlling the pulse currents supplied from the gradient magnetic field power source 4 to the x, y, z coils 3x to 3z, the gradient magnetic fields in the three-axis X, Y, and Z directions are synthesized and the slice direction gradient magnetic field G is synthesized._S, Phase encoding direction gradient magnetic field G_E, And readout direction (frequency encoding direction) gradient magnetic field G_REach direction can be set and changed arbitrarily. Each gradient magnetic field in the slice direction, the phase encoding direction, and the readout direction is represented by a static magnetic field H.₀Is superimposed on.
[0039]
The transmission / reception unit includes an RF (high frequency) coil 7 disposed in the vicinity of the patient P in the imaging space in the magnet 1, and a transmitter 8T and a receiver 8R connected to the RF coil 7. Under the control of a sequencer described later, the transmitter 8T supplies an RF current pulse having a Larmor frequency for exciting magnetic resonance (NMR) to the RF coil 7, while the receiver 8R receives the RF coil 7. The received MR signal (high frequency signal) is received, and various received signal processing is performed on the received signal to form corresponding digital data.
[0040]
The control / arithmetic unit further includes a sequencer (also referred to as a sequence controller) 5, a host computer 6, an arithmetic unit 10, a storage unit 11, a display device 12, and an input device 13. Among these, the host computer 6 has a function of commanding the pulse sequence information to the sequencer 5 according to the stored software procedure and overseeing the operation of the entire apparatus including the sequencer 5. An example of scan control by the host computer 6 will be described later.
[0041]
This MRI apparatus is characterized by performing an ECG-synchronized imaging scan at a preset ECG-synchronization timing. Specifically, while executing the main program, the host computer 6 performs a preparatory MR scan and an imaging MR scan as shown in FIG. The preparatory MR scan is a scan that executes a preparatory sequence for determining the synchronization timing used in the subsequent MR scan for imaging to an appropriate value in advance, and is hereinafter referred to as an “ECG-prep scan”. The MR scan for imaging is a scan that executes a sequence based on the electrocardiogram synchronization method at the synchronization timing set by the previous ECG-prep scan, and is simply referred to as “imaging scan” hereinafter. An example of an ECG-prep scan execution routine is shown in FIG. 3, and an example of an electrocardiographic imaging scan execution routine is shown in FIG. Although not shown in FIG. 2, a positioning scan is performed before the ECG-prep scan, and the imaging position is positioned based on the scan result.
[0042]
The sequencer 5 includes a CPU and a memory, stores pulse sequence information sent from the host computer 6, and controls a series of operations of the gradient magnetic field power source 4, the transmitter 8T, and the receiver 8R according to this information. . Here, the pulse sequence information is all information necessary for operating the gradient magnetic field power source 4, the transmitter 8T, and the receiver 8R according to a series of pulse sequences, for example, x, y, z coils 3x to 3z. Includes information on the intensity, application time, application timing, and the like of the pulse current applied to. In addition, the sequencer 5 inputs digital data (MR signal) output from the receiver 8 </ b> R and transfers this data to the arithmetic unit 10.
[0043]
This pulse sequence may be a two-dimensional (2D) scan or a three-dimensional (3D) scan as long as the Fourier transform method can be applied. In addition, the pulse train forms are SE (spin echo) method, FE (field gradient echo) method, FSE (fast SE) method, EPI (echo planar imaging) method, Fast asymmetric SE (FASE: half of FSE method) A method that combines the Fourier method) can be applied.
[0044]
Further, when imaging is performed with these pulse sequences, the imaging time per one RF excitation (that is, one shot) is on the order of about 200 to 1000 msec. For example, when the one-shot-FASE method (2D or 3D) is used, the imaging time in this range is achieved by employing the following imaging parameters as an example.
[0045]
[Outside 1]

[0046]
Further, the arithmetic unit 10 inputs the digital data of the MR signal sent from the receiver 8R via the sequencer 5, and the original data (also called raw data) to the Fourier space (also called k space or frequency space). And a two-dimensional or three-dimensional Fourier transform process for reconstructing the original data into a real space image, while performing an image data synthesis process. The Fourier transform process may be assigned to the host computer 6.
[0047]
A suitable example of this image data combining process is a process of adding reconstructed image data of a plurality of frames for each corresponding pixel, or a maximum value for selecting a maximum value for each corresponding pixel between the reconstructed image data of a plurality of frames. Value projection (MIP) processing. The addition processing includes simple addition processing, addition averaging processing, weighted addition processing, and the like. As another example of this combining process, the axes of a plurality of frames may be aligned in the Fourier space, and the original data may be combined into one frame of original data.
[0048]
The storage unit 11 can store not only the original data and the reconstructed image data but also the image data that has been subjected to the above-described combining process. The display device 12 displays an image. In addition, information such as parameter information for selecting a synchronization timing desired by the operator, scanning conditions, a pulse sequence, and an image composition method can be input to the host computer 6 via the input unit 13.
[0049]
Moreover, the audio | voice generator 19 is provided as a breath-hold command part. This voice generator 19 can emit, for example, messages indicating the start and end of breath holding as voice when instructed by the host computer 6.
[0050]
Further, the electrocardiogram measurement unit attaches to the body surface of the patient P and detects an ECG signal as an electrical signal, and performs various processing including digitization processing on the sensor signal to perform the host computer 6 and the sequencer. And an ECG unit 18 for outputting to the PC. The measurement signal from the electrocardiogram measurement unit is used by the host computer 6 and the sequencer 5 when executing the ECG-prep scan and the electrocardiogram synchronization imaging scan. Thereby, the synchronization timing of the electrocardiographic synchronization method can be appropriately set, and the MR original (raw) data can be collected by performing an electrocardiographic synchronization imaging scan based on the set synchronization timing.
[0051]
Next, processing for presetting the synchronization timing will be described with reference to FIGS.
[0052]
The host computer 6 starts an ECG-prep scan execution routine shown in FIG. 3 in response to a command from the input device 13 while executing a predetermined main program (not shown).
[0053]
First, the host computer 6 reads scan conditions and parameter information for executing the ECG-prep scan from the input device 13 (step S1 in the figure). Scan conditions include the type of scan, the type of pulse sequence, the phase encoding direction, and the like. The parameter information includes an initial time To for determining the ECG synchronization timing (here, a delay time from the peak value of the R wave in the ECG signal), a time increment increment Δt, an upper limit value of the number counter CNT, and the like. These parameters are included and can be arbitrarily set by the operator.
[0054]
Next, the host computer 6 clears the number counter CNT for counting the number of executions of the sequence, the time increment parameter Tinc for determining the synchronization timing, and the delay time TDL (CNT = 0, Tinc = 0, TDL = 0: Step S2). Thereafter, the host computer 6 sends message data to the voice generator 19 to cause the patient P to give a breath holding command such as “please hold your breath” (step S3). This breath holding is preferably performed in order to suppress the patient's body movement during the execution of the ECG-prep scan. However, in some cases, the ECG-prep scan may be executed without performing the breath holding. Good.
[0055]
When the preparation is completed in this way, the host computer 6 sequentially executes the processes after step S4. Thereby, it shifts to scan execution while changing the electrocardiogram synchronization timing.
[0056]
Specifically, the delay time TDL from the peak arrival time of the R wave is calculated by TDL = To + Tinc (step S4). Next, the ECG signal processed by the ECG unit 18 is read, and it is determined whether or not the peak value of the R wave in the signal has appeared (step S5). This determination process is repeated until the R wave appears. When the R wave appears (step S5, YES), it is subsequently determined whether or not the current delay time TDL calculated in step S4 has elapsed from the R wave peak time (step S6). This determination process is continued until the delay time TDL elapses.
[0057]
When the delay time TDL elapses from the peak time of the R wave (step S6, YES), the sequencer 5 is instructed to start a pulse sequence for each scan (step S7: see FIG. 4). The type of this pulse sequence is preferably set to be the same as an imaging pulse sequence described later. However, since the purpose of this ECG-prep scan is to set a delay time for electrocardiographic synchronization in which the signal intensity of the imaging target is the highest, even when the electrocardiographic synchronous imaging scan is performed in three dimensions. The ECG-prep scan may be performed in two dimensions as long as it includes the imaging target. This two-dimensional scan can minimize the entire imaging time. When the electrocardiogram-synchronized imaging scan is two-dimensional, the ECG-prep scan may be a two-dimensional scan, and the spatial resolution (matrix size, etc.) may be slightly reduced compared to the imaging scan.
[0058]
As this ECG-prep scan, a two-dimensional FASE (2D-FASE) method combining a high-speed SE method and a half Fourier method is used. Of course, instead of this scanning method, various two-dimensional scanning methods such as a normal SE method, a high-speed SE method, an EPI method, an FE method, and a segmented FFE method may be employed.
[0059]
In response to this sequence start command, the sequencer 5 starts executing a pulse sequence of the type commanded by the operator, so that the cross section of the desired part of the patient P is two-dimensionally scanned.
[0060]
After the sequence execution start command, the number counter CNT = CNT + 1 is calculated (step S8), and the time increment parameter Tinc = ΔT · CNT is calculated (step S9). As a result, the count value of the number counter CNT is incremented by 1 each time the execution of the pulse sequence is commanded, and the increment parameter Tinc for adjusting the synchronization timing is increased in proportion to the count value.
[0061]
Next, the process waits as it is until a predetermined period required for execution of each pulse sequence (for example, about 700 msec under the imaging conditions described later) elapses (step S10). Further, it is determined whether or not the number counter CNT has reached a predetermined upper limit value (step S11). In order to optimize the synchronization timing, the number counter CNT = 5 is set when, for example, five two-dimensional images are taken while changing the delay time TDL to various time values. If the number counter CNT has not reached the upper limit (step S11, NO), the process returns to step S4 and the above-described process is repeated. On the contrary, if the number counter CNT has reached the upper limit value (step S11, YES), a breath release cancellation command is issued to the sound generator 19 (step S12), and the subsequent processing is returned to the main program. The voice message for releasing the breath hold is, for example, “You can breathe.”
[0062]
When the above-described processes are sequentially executed, for example, a preparation pulse sequence is executed every time at the timing shown in FIG. For example, if the initial time To = 300 msec and time increment ΔT = 100 msec are commanded, the delay time TDL = 300 msec for the first sequence, the delay time TDL = 400 msec for the second sequence, and the third time The delay time TDL for determining the synchronization timing is adjusted such that the delay time TDL = 500 msec,. For this reason, when the first R wave after the breath holding command reaches the peak value, after the delay time TDL (= To) from the arrival time, for example, the first scan ECGprep1 based on the 2D-FASE method is performed for a predetermined time (about 700 msec) and echo signals are collected. Even when the next R wave appears during the continuation of the sequence, the sequence is continued without being involved in the appearance of the R wave because there is a standby process in step S10 of FIG. That is, the execution process of the sequence started in synchronization with a certain heartbeat is continued over the next heartbeat, and echo signals are collected.
[0063]
Then, when the number counter CNT has not reached the predetermined value, the processing from step S5 to step S11 in FIG. 3 is executed again. Therefore, in the example of FIG. 4, when the third R wave appears and reaches the peak value, the second scan ECGprep2 is performed for a predetermined time when the delay time TDL = To + Tinc = 400 msec has elapsed since this arrival time. The echo signal is collected as well. When this scan is finished and the next R wave appears, when the delay time TDL = To + 2 · Tinc = 500 msec elapses, the third scan ECGprep3 continues for a predetermined time, and echo signals are collected in the same manner. Further, when the next R wave appears after the end of this scan, when the delay time TDL = To + 3 · Tinc = 600 msec elapses, the fourth scan ECFprep3 continues for a predetermined time, and echo signals are similarly collected. This scan is continued a desired number of times, for example, 5 times, and echo data of the same cross section for a total of 5 frames (sheets) is collected.
[0064]
The echo data of each frame is sequentially sent to the arithmetic unit 10 via the sequencer 5. The arithmetic unit 10 reconstructs the echo data in the frequency space into an image in the real space by a two-dimensional Fourier transform method. This reconstructed image is stored in the storage unit 11 as MRA image data. For example, in response to an operation signal from the input unit 13, the host computer 6 sequentially displays the MRA images in cine.
[0065]
5A to 5E show display examples of a plurality of MRA images in which the electrocardiographic synchronization delay time (synchronization timing) is dynamically changed as described above. These figures are reproductions of actual image photographs. 2D-FASE method (effective TE (TEeff) = 40 ms, echo interval (ETS) = 5 ms, shot number = 1, slice thickness (ST) = 40 mm, number of slices (NS) = 1, number of sheets added (NAQ) = 1, matrix size = 256 × 256, FOV = 40 × 40 cm, actual scan time = about 700 ms), and phase encoding direction = vertical direction of the figure (body axis direction) ) Is a schematic representation of an image photograph of a lung field that was set and experimented. The blood flow as the target entity in this image is the descending aorta. In the figure, delay times TDL are respectively TDL = 300 msec in (a), TDL = 400 msec in (b), TDL = 500 msec in (c), TDL = 600 msec in (d), TDL = 700 msec in (e), and so on. It has become.
[0066]
When these cine display images are visually observed, the echo signal from the aortic flow appears most strongly and the entire aorta is clear in the MRA image of FIG. In the other MRA images (a) to (d), the range in which the aortic flow is reflected is very small or short compared to (e), and the velocity of blood flow accompanying pulsation is high. Due to factors such as being fast, the intensity of the echo signal is relatively low, and is close to the flow void phenomenon. That is, when obtaining an MRA image of the aortic flow in the lung field, in the case of this experiment, the state shown in FIG. 5E, that is, the delay time TDL = 700 msec is optimal. As a result, the synchronization timing of the electrocardiographic synchronization is found to be a time after the delay time TDL = 700 msec from the arrival time of the peak of the R wave.
[0067]
Therefore, the operator determines the optimum image, that is, the optimum delay time TDL by visual determination from the plurality of MRA images picked up by dynamically changing the delay time TDL in this manner, and performs the delay time parameter continuously. Perform the process of manually reflecting in the scan. Note that the term “optimum” here means “most likely” under the condition of the setting method given the synchronization timing.
[0068]
Furthermore, in the ECG-prep scan described above, the phase encoding direction is intentionally set to a direction (body axis direction) along the traveling direction of the aortic flow. As a result, compared to the case where the phase encoding direction is set to any other direction, it is possible to capture more clearly without missing or dropping the traveling direction information (directionality) of the aortic flow, and the imaging performance is excellent. ing. The reason for this will be described below.
[0069]
In general, blood flow represented by pulmonary blood vessels and liver blood vessels (portal veins) is T₂Time is slightly shorter (T₂= 100 to 200 ms). This T₂The shorter blood flow is T₂CSF and joint fluid (T₂It has been found that the full width at half maximum of the signal is wider than (> 2000 ms). This can be seen, for example, in the literature “R. Todd Cons-table and John C. Gore,“ The loss of small objects in Variable TE imaging—Fplication, FARE, RARE, and EPI ”, Magnetic Resonance in Medicine 28. , 9-24, 1992 ". According to the document, T₂As shown in FIG. 6, the spread of signal values for substances having different times is represented by “point spread function”. The graph in the figure is for a static magnetic field = 1.5 T, TEeff = 240 ms, echo interval (ETS) = 12 ms, the horizontal axis represents the number of pixels on the image in the phase encoding direction, and the vertical axis represents an arbitrary unit. Signal strength. According to this, T₂= T compared to 2000ms CSF and synovial fluid₂= 200 ms of blood (artery) has its full width at half maximum. This is T₂= 200 ms of blood (artery) can be said to be equivalent to the fact that the width in the phase encoding direction is apparently larger than that of CSF and joint fluid. Therefore, T₂= 200 ms of blood (artery) indicates that the entire image is more blurred in the phase encoding direction than CSF or synovial fluid.
[0070]
Therefore, by setting the phase encoding direction to the blood flow direction, T₂The degree of spread (blurring) on the pixel of the signal value in the phase encoding direction of blood with a short time is expressed as T₂The fact that the time is larger than the long one can be actively used, and the direction of blood flow is emphasized. Therefore, as described above, when selecting an optimal (appropriate) MRA image (that is, an optimal (appropriate) delay time) for ECG synchronization, the selection is facilitated.
[0071]
Next, the operation of the electrocardiographic synchronization imaging scan of this embodiment will be described with reference to FIGS.
[0072]
The host computer 6 executes the processing shown in FIG. 7 in response to the operation information from the input device 13.
[0073]
More specifically, the host computer 6 inputs the optimum (appropriate) delay time TDL determined by the operator through the ECG-prep scan described above via the input unit 13 (step S20). Next, the host computer 6 scans the conditions specified by the operator from the input device 13 (image size, number of scans, waiting time between scans, pulse sequence corresponding to the scan region, etc.) and information on the image composition processing method (reconstructed image). Or addition in frequency space, addition processing or maximum value projection (MIP) processing, etc. In the case of addition processing, any of simple addition, addition averaging processing, weighted addition processing, etc.) is input The information is processed into control information, and the control information is output to the sequencer 5 and the arithmetic unit 10 (step S21).
[0074]
The host computer 6 automatically changes the phase encoding direction according to the number of scans for achieving image composition (that is, how many images are captured in the same imaging region) in the process of step S21. The angle is calculated, and angle change information in the phase encoding direction for each scan is incorporated into the pulse sequence and sent to the sequencer 5. The angle change information is, for example, that when the number of images to be combined is two, the phase encoding direction is changed by 90 ° from the first time when the second scan is executed after the first scan is completed. It is.
[0075]
Next, when it is determined that there is an instruction for completion of preparation before scanning (step S22), a command to start breath holding is output to the sound generator 19 in step S23 (step S23). As a result, the voice generator 19 issues a voice message such as “please hold your breath” in the same way as in the ECG-prep scan, so that the patient who hears it stops breathing (see FIG. 9). .
[0076]
After instructing the start of breath holding, the host computer 6 stands by for a predetermined adjustment time Tsp (for example, 1 second) to estimate the timing at which the patient has completely shifted to the breath holding state (step S24).
[0077]
When waiting for this adjustment time is completed, the host computer 6 sequentially executes processing relating to the ECG signal (steps S25 to S27). First, an ECG signal is input, and the system waits while monitoring the ECG signal until an R wave peak value appears in the signal. When the R wave appears and reaches its peak, a process of waiting for the delay time TDL read in step S20 from the peak arrival time is performed (step S27). The value of this delay time TDL is optimal for the value that provides the highest echo signal intensity when imaging the target blood flow and tissue by ECG-prep pre-scanning, as described above, and is excellent in the imaging ability of the imaging target. It has become.
[0078]
The adjustment time Tsp ′ from the breath holding command to the first R wave appearance is a value obtained by adding the arbitrary time β from the time Tsp to the R wave appearance after the above-described value Tsp.
[0079]
The host computer 6 instructs the sequencer 5 to start scanning, assuming that the optimal delay time TDL has passed is the ECG synchronization timing (step S28). Upon receiving this command, the sequencer 5 drives the transmitter 8T and the gradient magnetic field power source 4 in accordance with the pulse sequence information that has already been sent and stored, and executes an imaging scan. An example of this scanning process is shown in FIG. 8, and the timing is shown in FIG.
[0080]
In the processing example shown in FIG. 8, the number of times of scanning is two, and the image composition processing described later adds two reconstructed images to each other. An example of this scan control will be described.
[0081]
The sequencer 5 normally stands by while determining whether an imaging scan start command has been sent from the host computer 6 (step S28-1). When the scan is commanded, the sequencer 5 executes the first scan based on the commanded phase encoding direction (step S28-2). In the case of this first scan, for example, the 2D-FASE method is selected, the phase encoding direction is set in the Z-axis direction, and the reading direction (frequency encoding direction) is set in the X-axis direction (see FIG. 9). As a result, for example, one frame of MR original data (raw data) associated with the lung field scan is collected.
[0082]
At this time, an echo signal generated from the patient P by the 2D-FASE method is received by the RF coil 7 and sent to the receiver 8R. In the receiver 8R, various kinds of preprocessing are performed on the echo signal and converted into a digital quantity. This digital amount of echo data is sent to the arithmetic unit 10 and arranged in, for example, a two-dimensional k-space based on the built-in memory. The set of echo data in the k space is converted into a real space tomographic image by, for example, two-dimensional Fourier transform at an appropriate timing. This reconstructed image data is temporarily stored in the storage unit 11 and waits for the second scan.
[0083]
After the first scan command, the sequencer 5 stands by while determining whether or not the scan has been completed (step S28-3).
[0084]
Thereafter, the sequencer 5 waits for a predetermined time Tw until the second scan (step S28-4). This waiting time Tw is intended to wait until the behavior of the nuclear spin based on the first scan returns to the steady state before the excitation pulse is applied. As a result, the behavior of the nuclear spin at the time of the second scan is hardly affected by that at the first time, so that the influence of the T1 relaxation time is somewhat lessened and the variation in echo data associated with the second scan is reduced. The waiting time Tw is, for example, on the order of about 6 seconds. Note that it is also a desirable aspect that the surgeon adjusts the length of the waiting time Tw via the input device 13.
[0085]
When this waiting time Tw elapses, the sequencer 5 inputs an ECG signal and waits for the appearance of an R wave peak therein (steps S28-5, 6). Therefore, the actual standby time Tw ′ is a value obtained by adding an arbitrary time β from the elapse of the time Tw to the appearance of the R wave to the designated standby time Tw. When the R wave appears, it again enters a standby state during the optimized delay time TDL (step S28-7).
[0086]
After the delay time TDL has elapsed, the sequencer 5 similarly executes the second scan for the same slice as the first time (step S28-8). However, the phase encoding direction at this time is changed by a preset angle and scanning is performed. For example, the phase encoding direction at the time of the second scanning is set in a direction shifted by 90 ° from the first phase encoding direction. As an example, the phase encoding direction is changed to the X axis direction, and the reading direction (frequency encoding direction) is changed to the Z axis direction. A second scan is performed in this encoding state (see FIG. 9), and the processing of the collected echo signals is the same as the first time.
[0087]
When the sequencer 5 can determine the completion of the second scan, the sequencer 5 notifies the host computer 6 of the completion of the scan (steps S28-9 and S10).
[0088]
The host computer 6 waiting in step S29 in FIG. 7 receives a scan completion notification from the sequencer 5. Thereby, the host computer 6 outputs a breath holding release command to the sound generator 19 (step S30). For this reason, the voice generator 19 issues a voice message such as “It is fine to breathe” toward the patient, and the breath holding period ends (see FIG. 9).
[0089]
When this series of data collection processing is completed, the host computer 6 instructs the arithmetic unit 10 to perform composition processing and display of the reconstructed images A and B based on the two scans primarily stored in the storage unit 11. (Step S31). Since this composition processing method can be recognized by the input processing in step S21, image composition is performed by that method to generate one composite image C. As a synthesis processing method, in this case, addition processing for adding two images A and B for each pixel value and maximum value projection processing for two images A and B can be used. In the case of addition processing, any one of simple addition, average addition, and weighted addition is instructed, and is performed in accordance with that method. As a result, as schematically shown in FIG. 9, a composite image C is obtained from the two reconstructed MRA images A and B.
[0090]
As described above, according to the present embodiment, the synchronization timing (the delay time TDL described above) when performing an electrocardiogram-gated imaging scan is appropriately set using a plurality of heartbeats. An imaging scan is performed in a state where the generated echo signal is the highest. It is possible to reliably avoid a state in which the blood flow velocity is relatively slow or the echo signal intensity is relatively lowered or almost zero due to the flow void phenomenon. It is possible to provide a stable MRA image with high rendering ability at optimized synchronization timing. In addition, when a high-speed SE system pulse sequence is used, it is possible to naturally enjoy advantages in terms of susceptibility and distortion of form.
[0091]
This is compared with the conventional ECG synchronization method. As in the prior art, when an imaging sequence is executed after a certain delay time has always elapsed from the peak time of an electrocardiogram waveform, for example, the turbulent blood flow time zone that occurs immediately after the appearance of the R wave is avoided, A time zone in which the blood flow state is relatively stable can be selected and scanned. As a result, the influence of turbulent blood flow can be eliminated, and an echo signal in a stable blood flow state can be arranged in the center region in the phase encoding direction of the frequency space to increase the contrast of the reconstructed image. .
[0092]
However, a certain delay time is not always appropriate. For example, even when imaging a tissue or blood flow with a short lateral relaxation time using the ECG synchronization method, the difference in the individual object or the imaging region Furthermore, an appropriate synchronization timing should exist depending on the type of pulse sequence. If there is an excess or deficiency in this synchronization timing, the flow of blood ejected from the myocardium has not yet sufficiently reached the imaging site, or conversely, the flow of ejected blood has already passed through the imaging site, A flow void phenomenon in which no echo signal is generated occurs.
[0093]
Optimization of the synchronization timing at which the echo signal value is strongest when imaging a tissue or blood flow with a short lateral relaxation time using the SE system sequence having a relatively long imaging time and the ECG synchronization method. Conventionally, a method that takes this into consideration has not been known, but the present invention can solve such a situation. Since the imaging time for one excitation is long, it is practically difficult to scan multiple times with different synchronization timings (time phases) within one heartbeat, but ECG-prep scan is performed over multiple heartbeats as in the present invention. By thinking and assigning different synchronization timings to a plurality of R waves, scanning can be performed with dynamic synchronization timing changes. As a result, a plurality of preparative MRA images reflecting the difference in synchronization timing, having relatively good contrast, and avoiding the influence of turbulent flow generation immediately after the appearance of the R wave are obtained. Using this image, the synchronization timing can be set optimally (appropriately) in advance, so that the various effects described above can be enjoyed.
[0094]
In addition, since the synchronization timing is optimized in this manner, there is almost no need to perform imaging again, the operator's operational burden is reduced, patient throughput can be improved, and the patient's burden is further reduced. Is also reduced or suppressed.
[0095]
By the way, according to the present embodiment, a new composite image can be obtained from a plurality of images of echo data collected by changing the phase encoding direction. This composite image is based on the control of changing the phase encoding direction.₂Excellent blood flow visualization with a short relaxation time.
[0096]
This is because the effect of enhancing (blurring) the pixel values in the phase encoding direction described with reference to FIG. 6 is actively used, and a plurality of images obtained with this enhancement effect are combined. This will be schematically described with reference to FIG. As shown in the figure, there is a blood vessel B11 that branches from the blood vessel B1 in the orthogonal direction. For example, the phase encoding direction at the time of the first scan substantially coincides with the traveling direction of the blood vessel B1, and the phase at the time of the second scan. Assume that the encoding direction substantially coincides with the branched blood vessel B11. As shown in FIG. 9A, each pixel is equivalent to a pseudo extension of the signal value in the phase encoding direction due to the first scan, and substantially coincides with the phase encoding direction. The existing blood vessel B1 is emphasized due to the blur, and conversely, the blood vessel B11 in the direction orthogonal to this is blurred. However, since the phase encoding direction is changed by 90 ° in the second scan, this time, as shown in FIG. 5B, one blood vessel B1 is blurred, while the other blood vessel B11 is blurred. Is emphasized.
[0097]
In the above-described embodiment, the reconstructed images in FIGS. 5A and 5B are added (synthesized) for each pixel, and as shown in FIG. Both images of B11 remain without being lost. Moreover, although it is blurred in the phase encoding direction, in the case of addition processing, since the two images are added for each pixel, the advantage of the averaging method can also be enjoyed, and the signal value of the blood flow is increased at the same time, S / N is improved. Although only two orthogonal directions have been described in FIG. 10, even if the blood flow B1 is slightly deviated from the first phase encoding direction and the blood flow B11 is slightly deviated from that of the second time, this advantage is somewhat. You can enjoy it. Therefore, a blood vessel such as a pulmonary blood vessel that is running indefinitely and horizontally can be rendered with a high S / N and a high contrast of a substantial part without almost missing the running direction information, thereby improving the diagnostic ability. Can contribute.
[0098]
In the case of the conventional averaging method with a fixed phase encoding direction, an improvement in the S / N ratio can be expected. However, when the phase encoding direction is set in the direction shown in FIG. Due to the blur, it may be difficult to visually identify or disappear. When the phase encoding direction was set in the direction shown in FIG. 5B, the blood flow B1 faced the same problem. However, according to the present embodiment, such a state is avoided, and in particular, T fields such as lung fields and blood vessels in the liver₂It has become possible to render blood vessels with shorter time without reducing the amount of information in the direction of travel.
[0099]
Furthermore, in the case of the above-described embodiment, all the imaging scans are completed twice in one breath holding period. For this reason, it is possible to suppress the occurrence of body movement artifacts due to periodic movements of the lungs and the like, and to reduce the generation of body movement artifacts due to the positional deviation of the patient's body itself when performing breath-hold imaging multiple times. . Thereby, a high quality MR image with less artifacts can be provided.
[0100]
In addition, since a waiting time for waiting for spin recovery is set between the two scans, the second scan can be executed more accurately and a high-quality image can be provided.
[0101]
Furthermore, even if such a waiting time is set, in most cases, the first and second scans are about 1.5 seconds, and the waiting time is about 4 seconds. That's enough. Therefore, the duration of one breath holding of the patient can be short, and there is an advantage that the mental and physical burden concerning breath holding is light for children and elderly people.
[0102]
Second embodiment
A second embodiment of the present invention will be described. Note that the second to fifth embodiments relate to other examples of the electrocardiographic synchronization imaging scan using the delay time TDL optimized through the ECG-prep scan described above.
[0103]
The imaging scan of the first embodiment described above is a total of two scans with the phase encoding direction changed, but the present invention is not limited to this. Therefore, in the MRI apparatus according to the second embodiment, as shown in FIG. 11, a total of four imaging scans are sequentially performed every predetermined waiting time by changing the phase encoding direction, and thereby the phase encoding direction is 45 degrees. Obtains MR original data for 4 frames different from each other. Each imaging scan employs an ECG gate method as an electrocardiogram synchronization method, and the synchronization timing is determined by a delay time TDL optimized through an ECG-prep scan performed in advance. As an example of image processing, original data is reconstructed in each frame, and four reconstructed images are combined (addition processing or maximum value projection processing). This also makes it possible to obtain MR images with abundant blood vessel travel information based on finer angle control in the phase encoding direction equivalent to or higher than that of the above-described embodiment.
[0104]
That is, the number n of images to be added (synthesized) (that is, the number of changes in the phase encoding direction) may be n ≧ 2.
[0105]
Third embodiment
A third embodiment will be described with reference to FIGS. This embodiment relates to a three-dimensional electrocardiographic synchronization imaging scan.
[0106]
In the case of this three-dimensional electrocardiogram-synchronized imaging scan, a plurality of slice encoding amounts corresponding to a plurality of slice encoding amounts are obtained while changing the phase encoding direction and the reading direction for each scan (interleaving) with the slice direction unchanged. Scans are performed. As a specific example, FIG. 12 shows an example of a sequence of ECG-synchronized imaging scans (executed after the ECG-prep scan described above) instructed by the host computer 6 and the sequencer 5. In scanning according to each slice encoding amount, an ECG gate method and a breath-holding method are employed in addition to a method of changing the phase encoding direction. The electrocardiogram synchronization timing is determined by a delay time TDL that is optimized through an ECG-prep scan performed in advance. For example, as shown in FIGS. 13A and 13B, the volume region data collection when the abdomen is three-dimensionally imaged is performed in the order of RLse1, HFse1, RLse2, HFse2,..., RLsen, HFsen. The scanning corresponding to the above is performed 2n times (n is an integer of 2 or more), for example, by the 3D-FASE method.
[0107]
The scan RLse or HFse represents an ECG-synchronized single scan for each slice encoding amount by the slice encoding gradient magnetic field that provides the three-dimensional original data of the volume region. However, the phase encoding direction is different between the scan RLse and the scan HFse. In the case of the scan RLse, as indicated by the solid line arrow X1 in FIG. 13B, the phase encoding direction is set to the left and right RL directions of the patient's body. On the other hand, in the case of scan HFse, as indicated by the dotted arrow X2 in the figure, the phase encoding direction is set to the patient's vertical (head / leg) HF direction, which is 90 degrees different from the horizontal direction. The subscripts se1... Sen represent the amount of gradient magnetic field in slice encoding for each scan. In this exemplary sequence, the first and second electrocardiogram-synchronized scans are performed for the same slice encoding amount se1 (... sen), and this set of scans is sequentially repeated while changing the slice encoding amount. In the case of this three-dimensional scan, since the entire imaging time becomes relatively long, the breath holding is performed in a plurality of times.
[0108]
Image reconstruction is performed individually with one set of three-dimensional original data in which the phase encoding direction is set in the left and right RL directions, and another set of three-dimensional original data in which the phase encoding direction is set in the up and down HF directions. Is done. Both three-dimensional reconstruction data are synthesized for each pixel and become final three-dimensional MRA data.
[0109]
Even with this three-dimensional imaging, it is possible to obtain effects such as ensuring the directionality of the blood flow, starting with the improvement of the rendering ability by the optimized ECG synchronization timing, as in the above-described embodiment. .
[0110]
Fourth embodiment
A fourth embodiment of the present invention will be described with reference to FIGS. This example relates to a multi-slice imaging scan.
[0111]
FIG. 14 shows an example of an electrocardiographic synchronization imaging scan sequence commanded by the host computer 6 and the sequencer 5. In this sequence, as in the third embodiment, each method of control of the phase encoding direction, ECG gate method as an electrocardiogram synchronization method, and one breath holding method is employed. The synchronization timing by the ECG gate method is determined by a delay time TDL optimized through an ECG-prep scan performed in advance.
[0112]
For example, when the abdomen is imaged by four multi-slice imaging, as shown in FIG. 15, for example, two-dimensionally in the order of scans RL1, RL2, RL3, RL4, HF1, HF2, HF3, HF4,. Data is collected based on the FASE method. As in the third embodiment, the scan RL indicates that the phase encoding direction is the left and right RL direction, and the scan HF indicates that it is the vertical HF direction, and two-dimensional original data of each slice is generated by each scan. Two frames of reconstructed image data whose phase encoding directions are 90 degrees different from each other are synthesized for each pixel, and MRA image data of each slice plane is created. For this reason, the detection capability of the high blood flow direction is ensured. Naturally, the effects of the ECG gate method and the breath holding method described above are also exhibited in this multi-slice imaging.
[0113]
Note that the scan order of the multi-slice imaging may be changed in any order such as RL1, HF1, RL2, HF2,.
[0114]
In each of the above-described embodiments, the phase encoding direction is changed at the time of the imaging scan so that a plurality of scans are performed. However, the present invention does not necessarily change the phase encoding direction as such. . A plurality of scans are generated in a predetermined constant phase encoding direction to generate a plurality of sets of image data, and the plurality of sets of image data are averaged to obtain a set of image data. Also good.
[0115]
Fifth embodiment
A fifth embodiment of the present invention will be described with reference to FIGS. In the present embodiment, the present invention is applied when imaging a plurality of types of objects.
[0116]
In each of the above-described embodiments, the synchronization timing (delay time TDL) of the ECG gate method (electrocardiographic synchronization method) optimally set by the ECG-prep scan is one quantity, that is, one fixed synchronization timing.
[0117]
When there is only one type of imaging target, this single synchronization timing is still in time, but if the imaging target is the patient's aorta AR and liver portal PV as shown in FIG. 16, the former is almost along the body axis direction. The latter has many parts which run along the left-right direction orthogonal to the body axis direction. That is, it is generally assumed that when the types of blood vessels to be imaged are different, the traveling directions are different, and the optimum synchronization timing in the ECG gate method is also different. Therefore, the MRI apparatus of the present embodiment is characterized in that a plurality of ECG gate method synchronization timings are set in accordance with the type of imaging object, that is, the difference in the running direction of blood vessels and tissues.
[0118]
Specifically, as shown in FIG. 17, the host computer 6 and the sequencer 5 work together to sequentially execute a total of two ECG-prep scans prior to the electrocardiographic synchronization imaging scan. In the first ECG-prep scan # 1, for example, the ECG-prep scan shown in FIGS. 3 and 4 described above is executed by the two-dimensional FASE method with the phase encoding direction set to, for example, the vertical (body axis) HF direction. The With the first ECG-prep scan, in the example shown in FIG. 16, a plurality of time-phase scans are executed as described above in a state in which information collection in the traveling direction of the aorta AR is emphasized. As a result, the optimum delay time TDL = α1 at which the intensity of the MR signal from the aorta AR is maximized is set.
[0119]
In the second ECG-prep scan, the above-described ECG-prep scan shown in FIGS. 3 and 4 is executed by the two-dimensional FASE method with the phase encoding direction set to, for example, the left and right RL directions. According to the second ECG-prep scan, in the example shown in FIG. 16, a plurality of time-phase scans are executed as described above in a state in which information collection in the traveling direction of the portal vein PV is emphasized. As a result, the optimum delay time TDL = α2 at which the intensity of the MR signal from the portal vein PV is maximized is set.
[0120]
Thereafter, the host computer 6 and the sequencer 5 work together to execute an electrocardiographic synchronization imaging scan based on the three-dimensional FASE method using these two types of delay times TDL = α1 and α2. A sequence example of this imaging scan is shown in FIG. As shown in the figure, at the time of scanning for the first slice encoding amount se1 for the first time, scanning is performed in synchronization with the delay time TDL = α2 according to the running direction of the portal vein PV. In response to this, scanning is executed based on the three-dimensional FASE method in the phase encoding direction = left-right RL direction and the slice encoding amount se1. Further, in the second scan for the first slice encoding amount se1, the scan is performed in synchronization with the delay time TDL = α1 according to the traveling direction of the aorta AR. In response to this, the scan is executed based on the three-dimensional FASE method in the state of the phase encode direction = the vertical HF direction and the slice encode amount se1.
[0121]
Similarly, while changing the slice encoding amount se, the phase encoding direction based on the delay time TDL = α2 = the ECG-synchronized imaging scan in the left / right RL direction, and the phase encoding direction based on the delay time TDL = α1 = the upper and lower HF directions The ECG synchronization imaging scan is repeated alternately. The echo signals collected by this series of scans are processed and displayed in the same manner as in the first embodiment.
[0122]
As described above, the synchronization timing (delay time) for the ECG gating method is individually set according to the traveling direction of the two types of imaging targets by the individual ECG-prep scan, and the two amounts An electrocardiographic synchronization imaging scan is performed in a phase encoding direction in accordance with the traveling direction of two types of imaging targets (aorta and portal vein) based on each synchronization timing, and an MR image is generated.
[0123]
For this reason, not only the above-described effect when imaging in accordance with the phase encoding direction for each of the two types of imaging objects, but also the synchronization timing itself of the ECG gate method for each patient, each imaging object, and the pulse sequence used. It is set correspondingly. Therefore, for example, regardless of the blood flow velocity, an electrocardiographic synchronization imaging scan is performed in a state where the signal value from the target blood vessel is the largest, so that two types of imaging objects are reliably captured. That is, traveling information of all imaging targets is sufficiently ensured, and a high S / N MR image is provided with high rendering ability.
[0124]
Note that the above description is for the case where there are two types of imaging targets, the aorta and the portal vein, but the same applies to three or more types of imaging targets, and ECG-prep scans corresponding to the number of types are performed. What is necessary is just to set the synchronous timing for every imaging object. Further, the ECG-gated imaging scan using the plurality of synchronization timings is not limited to the three-dimensional imaging by the three-dimensional FASE method of FIG. 18, but the single slice imaging by the two-dimensional FASE method shown in FIG. The multi-slice imaging by the two-dimensional FASE method shown in FIG. Further, the pulse sequence used for the imaging is not limited to the FASE method, and may be a high-speed SE method or an FE-based pulse sequence.
[0125]
Sixth embodiment
A sixth embodiment of the present invention will be described with reference to FIGS.
[0126]
The MRI apparatus according to this embodiment employs a segmented FFE method (hereinafter referred to as a seg FFE method) suitable for cardiac system imaging as a pulse sequence. As an example, a case will be described in which an ECG-prep scan is performed by a two-dimensional segFFE method, and a subsequent electrocardiographic synchronization imaging scan is performed by a three-dimensional segFFE method. Thus, the measurement time of the electrocardiogram synchronization timing can be shortened by reducing the dimension of the ECG-prep scan.
[0127]
Specific examples of sequences employing this segFFE method are shown in FIGS. FIG. 19 shows an outline of the ECG-prep scan, FIG. 20 shows an example of a pulse sequence based on the two-dimensional segFFE method of the ECG-prep scan, and FIG. 21 shows an outline of the electrocardiographic synchronization imaging scan. FIG. 22 shows an example of a pulse sequence based on the three-dimensional seg FFE method of an electrocardiographic synchronization imaging scan. Although not shown in this ECG-prep scan and imaging scan, a breath holding method is used in combination.
[0128]
The ECG-prep scan employs a method called “single slice multiphase”. The host computer 6 instructs the sequencer 5 to use a two-dimensional segFFE method pulse sequence based on this method.
[0129]
According to this “single slice multiphase” method, echo data of a plurality of time phases can be collected at a time for a single slice determined by the gradient magnetic field GS for slice and the RF frequency. For this reason, the slice thickness is determined so that the blood vessel for imaging enters the slice.
[0130]
This will be described in detail. From the time when the delay time TDL = α1 has elapsed from the appearance time of the R wave peak value between the R and R waves of the ECG signal from which a single continuous data called a segment is obtained, FIG. As shown in phase 1 of segment 1, a plurality of sets of RF excitation and echo collection by the FE method are repeated (here, collection of four echo signals). A plurality of (four in this case) echo signals collected in this way are sent to the arithmetic unit 10 through reception processing. The arithmetic unit 10 divides the phase encoding direction into a plurality (here, four) and forms a plurality of k spaces (here, five), and the plurality of echo signals collected in the phase 1 are It is arranged on the first line of each divided area of the first k space KS1 according to the phase encoding amount (see FIG. 19).
[0131]
Further, from the time when the delay time TDL = α2 (> α1) has elapsed from the appearance time of the R wave peak value, as shown in the phase 2 of the segment 1, a plurality of sets of RF excitation and echo collection by the FE method are repeated. The echo signal obtained as a result is arranged on the first line of each divided region of the next k space KS2. For each of the delay times TDL = α3 (> α2), α4 (> α3), α5 (> α4), four echo signals are collected in the same manner, and the third, fourth, and fifth k-space KS3 are collected. , KS4, and KS5 are arranged on the first line of each divided region. As a result, echo data is arranged on the first line of each divided region in each of the five k spaces.
[0132]
Next, the same echo collection and placement is performed for the segment 2 between the next RR waves. However, the collected data at this time is arranged on the next line of each divided region in each k space. Thereafter, the same processing is performed for the segments 3, 4, and 5.
[0133]
Therefore, when this ECG-prep scan is completed, the data arrangement of all five k spaces is completed. The arithmetic unit 10 reconstructs five real space images by two-dimensional Fourier transform of these five k-space data. That is, it is possible to obtain a preparatory image with a delay time TDL = α1, α2, α3, α4, α5 at a time in one scan. Therefore, the operator visually observes these five preparation images, for example, and specifies an image in which the imaging region is most clearly displayed, that is, an optimum value of the delay time TDL.
[0134]
Next, at an appropriate timing, the host computer 6 instructs the sequencer 5 to perform a three-dimensional segFFE electrocardiographic synchronization scan as shown in FIGS. The optimum delay time TDL = αm used in this imaging scan is a value set in the above-described ECG-prep scan.
[0135]
The pulse train for each segment of this imaging scan includes an MT pulse Pmt that gives an MT (magnetization transfer) effect applied first, a chemical selection pulse Pchess for fat suppression applied next, and then each gradient magnetic field direction. The dephase spoiler pulses SPs, SPr, and SPe to be applied are included as a preparation pulse train. A data acquisition pulse train Pacq for collecting field echoes is arranged after the preparation pulse train. Since this imaging scan is a three-dimensional scan, a slice gradient magnetic field GE2 is applied in addition to the phase encoding gradient magnetic field GE1.
[0136]
The plurality of segmented echo signals are arranged in the three-dimensional k-space of the arithmetic unit 10 according to the slice encoding amount and the phase encoding amount. This arrangement data is then reconstructed into real space data by three-dimensional Fourier transformation, and further converted into a two-dimensional image by MIP processing, for example.
[0137]
As described above, according to the present embodiment, the scan method that scans a plurality of times by spatially changing the phase encoding direction is not employed as in the above-described embodiment, but the heart system is reliably imaged using the segFFE method. can do. In particular, since the ECG synchronization timing at this time is optimized in advance, it is possible to reliably capture the fast blood flow of the heart system.
[0138]
Since the ECG synchronization timing is measured by the “single slice / multiphase” method as described above, images of a plurality of time phases can be obtained at one time by one ECG-prep scan. That is, it is not necessary to perform a plurality of ECG-prep scans, the entire imaging time can be shortened, and patient throughput is improved.
[0139]
Note that the ECG-prep scan using the segFFE method is not limited to the above-mentioned “single slice / multiphase” method. For example, as shown in FIG. 23, the “multislice / single phase” method can be simplified. (Single phase here means one phase for each slice). As shown in the figure, a plurality of echo signals of slice 1 in each segment are collected to fill the k space KS1 of slice # 1. Also, a plurality of echo signals of slice 2 of each segment are collected to fill the k space KS2 of another slice # 2. Similarly, a plurality of echo signals of slices 3, 4, and 5 of each segment are collected to fill the k spaces KS 3, KS 4, and KS 5 of another slice # 3, # 4, and # 5, respectively. Thereby, images of a plurality of slices # 1 to # 5 having different acquisition time phases determined by the delay times TDL = α1, α2, α3, α4, and α5 are obtained by one scan of the segFFE method.
[0140]
Since the plurality of slices # 1 to # 5 are close to each other, it is safe to assume that the slices (images) reflect the ECG synchronization timing of the imaging region almost accurately. There are also many. In such a case, the plurality of images may be handled as one image, and an optimal delay time may be set from these images. In particular, when this simple “multi-slice single-phase” method can be used, it is more advantageous than the “single-slice multi-phase” method in terms of SAR (RF exposure).
[0141]
A method similar to the segFFE method can be applied to the EPI method. That is, the ECG-prep scan is performed by a two-dimensional EPI method, and an imaging scan based on the three-dimensional EPI method is executed using a delay time optimized thereby.
[0142]
Seventh embodiment
A seventh embodiment of the present invention will be described with reference to FIGS.
[0143]
The MRI apparatus according to this embodiment is characterized in that the selection process of the optimum delay time from the image data of a plurality of time phases obtained by the ECG-prep scan and the reflection process of the delay time in the imaging scan are saved. .
[0144]
In order to achieve this feature, the host computer 6 performs a series of processes shown in FIG. As in the above-described embodiments, when a preparation image having a plurality of time phases (delay times) is obtained by the ECG-prep scan, the host computer 6 proceeds to the processing of FIG. 24 and the plurality of preparation images IM1 to IM1. IM5 is displayed as shown in FIG. 25 (step S41). The value of the delay time TDL is also superimposed on each of the preparation images IM1 to IM5 (step S42).
[0145]
When this display is completed, the host computer 6 superimposes and displays the line ROI on the initial positions of the preparation images IM1 to IM5 as shown in FIG. 25 (step S43). Next, the input signal from the input device 13 is read, the ROI position is adjusted according to the operator's designation, and it is determined whether or not the ROI position is OK (steps S44 and S45). Thereby, the line ROI is set at a position on the blood vessel, for example, which the operator desires interactively with the operator.
[0146]
When this ROI setting is completed, the host computer 6 calculates the signal intensity distribution DS for the pixel at the designated line position on each preparation image IM, and displays the distribution DS superimposed on each image (steps S46 and S47). An example of this display is shown in FIG.
[0147]
The host computer 6 specifies an image (distribution) exhibiting, for example, the highest value of the signal intensity (pixel value) distribution (step S48). This “highest value” is one index for specifying an image (distribution) having the highest signal intensity at the ROI position, and other indices may be used. A rectangular ROI may be used instead of the line ROI, and a histogram may be used instead of the signal intensity distribution.
[0148]
Next, the host computer 6 determines the specified preparation image, that is, the time phase (delay time) TDL of the image as the optimum electrocardiographic synchronization timing, and stores the value in the internal memory (step S49). The stored delay time TDL is automatically read from the memory during the imaging scan. That is, in the process of step S50 corresponding to the process of step S20 in FIG. 7 described above, the optimum delay time TDL that has already been stored is automatically read instead of being manually designated.
[0149]
Thus, ECG-prep scan is performed by FASE method, EPI method, and segFFE method of two-dimensional scan with a short imaging time, and the optimal ECG synchronization timing is determined interactively with the operator, and the ECG synchronization timing is determined. It can be automatically reflected in the imaging scan.
[0150]
Accordingly, it is possible to reduce the labor when the operator determines a plurality of preparation images obtained by the ECG-prep scan, and to reduce the determination error, thereby obtaining a more accurate electrocardiographic synchronization timing. Thereby, the blood-capturing capability in the electrocardiogram synchronous imaging scan can be enhanced. In addition, since the determination result of the ECG synchronization timing based on the ECG-prep scan is automatically reflected in the ECG-synchronized imaging scan, the operation effort is remarkably reduced from this point.
[0151]
In the above-described embodiment, the most suitable image among the plurality of preparation images is selectively determined from the calculation result such as the signal intensity distribution, and the delay time TDL assigned to the image is determined as an appropriate synchronization timing. However, the method for determining the synchronization timing according to the present invention is not limited to this. In short, any synchronization timing may be selected or determined using data of a plurality of preparation images.
[0152]
For example, in the above-described embodiment, two or more preparation images are appropriately selected from a plurality of preparation images, and a known method such as a curve fitting method is used from a plurality of delay time values assigned to these selection images. Based on the above, an optimum delay time may be further calculated.
[0153]
For example, without displaying the preparatory image, the signal intensity distribution, histogram, etc. are automatically calculated from the image data, the calculation result is automatically determined by a predetermined algorithm, and the synchronization timing is automatically determined from the determination result. You may decide to. As described above, the synchronization timing is automatically reflected in the ECG-gate imaging scan, thereby eliminating the need for the operator to set the synchronization timing between the ECG-prep scan and the imaging scan.
[0154]
By the way, in each of the above-described embodiments and modifications thereof, MR angiography (MRA) has been aimed. However, the imaging target is not limited to only a blood vessel, but may be an arbitrary target such as a tissue running in a fibrous form. It's okay. In particular, T₂If the time is short, the ECG-prep scan and the subsequent imaging scan according to the present invention can be suitably performed.
[0155]
【The invention's effect】
  According to the MRI apparatus of the present inventionWhen imaging using the electrocardiographic synchronization method, multiple heartbeats are used regardless of the length of imaging time associated with one RF excitation (one shot).eachThrough the execution of the preparatory MR scan, the synchronization timing (time phase) of the ECG synchronization method is appropriately determined in advance. For this reason, the synchronization timing when performing an electrocardiogram-synchronized scan as an imaging scan is appropriate for an imaging target such as a blood flow flowing through the diagnostic region, and the intensity of the echo signal generated from the imaging target is the highest. As a result, it is possible to reliably avoid a state in which the intensity of the echo signal is relatively lowered or almost zero due to a so-called flow void phenomenon and the like, and the visual recognition that improves the rendering of the imaging target itself and the information amount thereof. A high-definition MR image with high image quality can be provided stably.
[0156]
In particular, for example T₂Time T₂The effect is remarkably exhibited when imaging a short blood of 100 to 200 ms. At the same time, by using the breath-holding method together with the preparatory scan, it is possible to prepare a high-quality MR image with few body motion artifacts, thereby further improving the setting accuracy of the synchronization timing of electrocardiographic synchronization.
[0157]
Further, according to the present invention, when optimizing the ECG synchronization timing, at least in the preparatory MR scan, the phase encoding direction is adjusted to the traveling direction of the imaging target. The traveling direction of the flow can be reliably depicted, and abundant traveling information can be provided.
[0158]
Furthermore, according to the present invention, the optimization of the electrocardiographic synchronization timing is reduced as much as possible by labor saving and automating as much as possible through interactive operation and automatic calculation with the operator, while the determined optimal electrocardiographic synchronization timing is set to the MR for imaging. Since it is automatically reflected in the scan, it is possible to reduce the labor and load on the operator's operation and contribute to the improvement of patient throughput.
[Brief description of the drawings]
FIG. 1 is a functional block diagram showing an example of the configuration of an MRI apparatus according to an embodiment of the present invention.
FIG. 2 is a view for explaining the time relationship between an ECG-prep scan and an electrocardiographic synchronization imaging scan in the embodiment.
FIG. 3 is a schematic flowchart illustrating an ECG-prep scan procedure executed by a controller.
FIG. 4 is a timing chart showing an example of an ECG-prep scan.
FIG. 5 is a diagram schematically showing a lung field MRA image obtained by ECG-prep scan when the delay time is dynamically changed.
FIG. 6 is a diagram for explaining the spread of signal values in the phase encoding direction.
FIG. 7 is a schematic flowchart illustrating a processing example of an imaging scan executed by a host computer in the embodiment.
FIG. 8 is a schematic flowchart showing an example of scan control processing executed by the sequencer in the embodiment;
FIG. 9 is a diagram schematically illustrating a relationship between an imaging can scan order and image composition in the embodiment;
FIG. 10 is a schematic diagram illustrating a pixel array reflecting an apparent spread of signal values in different phase encoding directions during single scanning and that during image synthesis.
FIG. 11 is a diagram schematically illustrating a relationship between an imaging scan order and image composition according to the second embodiment of the present invention.
FIG. 12 is a diagram schematically illustrating a relationship between a scan order of a three-dimensional imaging scan and an image composition according to the third embodiment of the present invention.
FIG. 13 is a diagram for explaining a relationship between a volume area and a gradient magnetic field direction setting related to a three-dimensional scan.
FIG. 14 is a diagram schematically illustrating the relationship between the scan order of multi-slice scanning and image composition according to the fourth embodiment of the present invention.
FIG. 15 is a view for explaining the positional relationship of slice planes related to multi-slice scanning.
FIG. 16 is a diagram illustrating setting of a phase encoding direction for each imaging target of an ECG-prep scan according to the fifth embodiment of the present invention.
FIG. 17 is a diagram for explaining a time relationship of two-stage ECG-prep scan.
FIG. 18 is a partial time chart for explaining an electrocardiogram synchronization imaging scan using two values of synchronization timing.
FIG. 19 shows seg. Of an ECG-prep scan according to the sixth embodiment of the present invention. The figure explaining the relationship of the sequence of FFE method, acquisition time phase, and echo data arrangement | positioning.
FIG. 20 shows a two-dimensional seg. The sequence explaining an example of the pulse train of FFE method.
FIG. 21 shows a seg. Of an imaging scan according to the sixth embodiment. The figure explaining the relationship of the sequence of FFE method, acquisition time phase, and echo data arrangement | positioning.
FIG. 22 shows a three-dimensional seg. The sequence explaining an example of the pulse train of FFE method.
FIG. 23 shows seg. Of an ECG-prep scan according to a modification of the sixth embodiment. The figure explaining the relationship of the sequence of FFE method, acquisition time phase, and echo data arrangement | positioning.
FIG. 24 is a schematic flowchart showing automation processing for determining optimum synchronization timing according to the seventh embodiment of the present invention.
FIG. 25 is a display state diagram of a preparatory image showing a process of determining the optimum synchronization timing.
FIG. 26 is a display state diagram of a preparatory image showing another process of determining the optimum synchronization timing.
[Explanation of symbols]
1 Magnet
2 Static magnetic field power supply
3 Gradient magnetic field coil unit
4 Gradient magnetic field power supply
5 Sequencer
6 Controller
7 RF coil
8T transmitter
8R receiver
10 Arithmetic unit
11 Storage unit
12 Display
13 Input device
16 Sound generator
17 ECG sensor
18 ECG units
19 Sound generator

Claims (26)

In an MRI apparatus for performing MR scan in synchronization with a signal representing a cardiac time phase of a subject collected by a signal collecting means for collecting a signal representing a cardiac time phase of the subject on a desired region of the subject ,
MR by performing a plurality of preparatory MR scans for each of a plurality of heartbeats with respect to the region of the subject at different synchronization times based on a plurality of reference waves of a signal representing a cardiac time phase of the subject. A preparatory scanning means for collecting each signal;
Preparation image generation means for generating a plurality of preparation MRA images according to the plurality of different synchronization times from the MR signal;
A preparation information reflecting means for reflecting the information about synchronization time collected certain preparatory MRA image of the plurality of preparatory MRA image in MR scanning MRA imaging,
An MRI apparatus comprising: control means for executing the MR scan for MRA imaging based on information on the synchronization time reflected by the preparation information reflecting means. In the invention of claim 1,
The preparation information reflecting unit includes display means for displaying the preparatory MRA image of the plurality, a selection means for selecting a desired preparation for MRA image of the displayed plurality of prep MRA image, the An MRI apparatus comprising: reflection means for reflecting, on the MR scan for MRA imaging, information relating to the synchronization time of the MR scan for preparation collected to obtain the MRA image for preparation selected by the selection means. In the invention of claim 1,
The preparation information reflecting unit includes display means for displaying the preparatory MRA image of the plurality, a manual designation means for designating a desired position of the displayed plurality of prep MRA images manually, the at the specified position automatically selecting a calculation means for automatically calculating the information related to the intensity of the data ready for MRA image, the desired preparations for MRA image of the plurality of preparatory MRA image according to the result of this calculation means An MRI apparatus comprising: a selection unit that automatically reflects the synchronization time of the selected preparation MRA image as the synchronization time for the electrocardiographic synchronization method in the MR scan for MRA imaging . The imaging target of the subject is captured at different synchronization times based on a plurality of reference waves of the signal representing the cardiac time phase of the subject collected by the signal collecting means for collecting the signal representing the cardiac time phase of the subject. Preparatory scanning means for collecting MR signals by executing a preparatory MR scan a plurality of times for a plurality of heartbeats with respect to a region including
Preparation image generating means for generating a plurality of preparation MRA images corresponding to the plurality of different synchronization times from the MR signal;
Synchronization time specifying means for specifying the synchronization time of MR scan for MRA imaging based on information of the plurality of MRA images for preparation;
An MRI apparatus comprising: an imaging scan unit that executes an MR scan for MRA imaging of the region of the subject in synchronization with the specified synchronization time. In the invention of claim 4,
The MRI apparatus is a means for collecting the ECG signal of the subject as a signal representing the cardiac time phase. In the invention of claim 5,
The preparation scanning means includes a changing means for changing a delay time from an R wave as a reference wave included in the ECG signal to a plurality of time values, and the elapsed time of each changed delay time as the synchronization time. An MRI apparatus comprising: a scan start means for starting the preparation MR scan. In the invention of claim 6,
The MRI apparatus in which the imaging target of the subject to be scanned by the preparation scanning unit is a tissue or blood flow with a short transverse relaxation time of nuclear spin. In the invention of claim 6,
The MR scan for preparation and the MR scan for MRA imaging are both MR scans using the same kind of pulse sequence. In the invention of claim 8,
The pulse sequence is a high-speed SE method pulse sequence or a pulse sequence using this high-speed SE method. In the invention of claim 9,
The pulse sequence is an MRI apparatus in which the scan time per one RF excitation is about 200 msec or more. In the invention of claim 10,
The pulse sequence is a sequence that depends on a Fourier transform method in which the MR signal is arranged in a frequency space and subjected to Fourier transform to reconstruct a real space image, and the frequency space is divided into predetermined amounts in the phase encoding direction. An MRI apparatus which is a sequence in which the MR signals are arranged by dividing the MR signal. In the invention of claim 8,
The MRI apparatus is a pulse sequence of a segmented high-speed FE method. In the invention of claim 12,
The pulse sequence of the segmented fast FE method collects a plurality of echo signals at each of a plurality of different times between the R waves, and a plurality of two-dimensional corresponding to a single slice passing through the region of the subject. An MRI apparatus having a pulse train that arranges the echo signals at the same time in each of k spaces. In the invention of claim 8,
The MRI apparatus, wherein the pulse sequence is an EPI pulse sequence. In the invention of claim 1, 4 or 8,
The MR scan for preparation uses a pulse sequence of a two-dimensional scan, and the MR scan for MRA imaging uses a pulse sequence of a three-dimensional scan. In the invention of claim 4,
The preparation scanning means is configured to prepare the plurality of types of blood vessels to be imaged by substantially matching the traveling direction of the blood vessels branching the phase encoding direction in a plurality of scans executed by changing the phase encoding direction . Execution means for executing MR scan for each imaging target;
The preparation image generating means is means for generating the plurality of preparation MRA images from the MR signals collected by the preparation MR scan for each imaging target,
The synchronization time specifying means is means for specifying the desired synchronization time based on information of the plurality of preparation MRA images for each imaging target,
The imaging scanning unit performs MR scan for MRA imaging in synchronization with the desired synchronization time specified for each imaging target in a phase encoding direction substantially matched with a traveling direction of a plurality of types of blood vessels of the imaging target. An MRI apparatus comprising execution means for executing for each imaging target. In the invention of claim 16,
The MRI apparatus, wherein the plurality of types of imaging targets are a plurality of types of bloodstreams that travel in spatially different directions within the region of the subject. In the invention of claim 17,
The imaging scanning means includes collecting means for collecting echo signals collected by the MRA imaging MR scan for each imaging target, and generating means for generating image data of the region from the echo signals for each imaging target. MRI equipment. In the invention of claim 18,
The MRI apparatus comprises: means for reconstructing the echo signal into real space image data for each imaging target; and means for synthesizing the reconstruction data for each imaging target for each pixel. In the invention of claim 4,
The synchronization time specifying means includes an interface means for presenting the plurality of MRA images for preparation to the operator and receiving designation of a portion desired by the operator, and the desired synchronization time from data on the intensity of the echo signal of the portion of interest. An MRI apparatus comprising a confirming means for confirming. In the invention of claim 20,
The synchronization time specifying means further comprises an MRI apparatus the reflection means to automatically reflect the determined said desired synchronization time to the sequence of the MRA imaging MR scan. In the invention of claim 20,
The MRI apparatus includes a display unit for displaying the information indicating the plurality of different synchronization times on the plurality of preparation MRA images, respectively, by superimposing the information on the plurality of preparation MRA images . In the invention of claim 20,
While the interface means includes ROI designation means for designating a part desired by the operator by ROI,
The MRI apparatus includes means for calculating a signal intensity distribution of the ROI portion from a pixel value corresponding to the portion and means for determining the desired synchronization time from the signal intensity distribution by a predetermined algorithm. . In the invention of claim 4,
The MRI apparatus, wherein the preparation scanning means includes means for setting a phase encoding direction used for the preparation MR scan in accordance with a traveling direction of the imaging target. In the invention of claim 4,
An MRI apparatus comprising breath holding command means for commanding the subject to perform breath holding while the preparation MR scan is executed by the preparation scanning means. In an MRI apparatus for performing MR scan in synchronization with a signal representing a cardiac time phase of a subject collected by a signal collecting means for collecting a signal representing a cardiac time phase of the subject on a desired region of the subject ,
MR signals are collected by executing multiple preparatory MR scans for each of a plurality of heartbeats for the region of the subject at different delay times from a reference wave of a signal representing the cardiac phase of the subject. Scanning means for preparation
Preparation image generating means for generating a plurality of preparation MRA images corresponding to the plurality of different delay times from the MR signal;
Preparation information reflecting means for reflecting the information of the plurality of MRA images for preparation to the MR scan for MRA imaging;
Display means for displaying the plurality of preparation MRA images provided in the preparation information reflecting means ;
Selecting means for selecting a desired preparation MRA image from the displayed plurality of preparation MRA images ;
Control means for executing MR scan for MRA imaging based on information relating to delay time of MR scan for preparation collected to obtain the MRA image for preparation selected by the selection means;
An MRI apparatus characterized by comprising:

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